Remove vendored pip-grep script (#1116)

Since the last usage was removed in #1113, and pip-grep is actually
broken on newer pip anyway (though helpfully silently ignores
exceptions, so one wouldn't know).

This is the first of the pip-pop removals, the rest will follow as the
last usages are switched over.

Whilst this tool was intended for internal buildpack usage only, the
vendor directory is on `PATH`, so I've documented the removal in
CHANGELOG just in case.

Refs @W-8208817@.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Remove vendored `pip-grep` script (#1116).
 
 ## v185 (2020-11-12)
 

vendor/pip-pop/pip-grep

@@ -1,94 +0,0 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-
-"""Usage:
-  pip-grep [-s] <reqfile> <package>...
-
-Options:
-  -h --help     Show this screen.
-"""
-import os
-import sys
-from docopt import docopt
-
-try:  # pip >= 10
-    from pip._internal.req import parse_requirements
-    from pip._internal.download import PipSession as session
-
-    def PackageFinder(find_links, index_urls, session=None):
-        from pip._internal.index import PackageFinder
-        from pip._internal.models.search_scope import SearchScope
-        from pip._internal.models.selection_prefs import SelectionPreferences
-
-        search_scope = SearchScope.create(find_links, index_urls)
-        selection_prefs = SelectionPreferences(allow_yanked=False)
-        return PackageFinder.create(search_scope, selection_prefs, session=session)
-
-except ImportError:  # pip <= 9.0.3
-    from pip.req import parse_requirements
-    from pip.index import PackageFinder
-    from pip._vendor.requests import session
-
-
-requests = session()
-
-
-class Requirements(object):
-    def __init__(self, reqfile=None):
-        super(Requirements, self).__init__()
-        self.path = reqfile
-        self.requirements = []
-
-        if reqfile:
-            self.load(reqfile)
-
-    def __repr__(self):
-        return '<Requirements \'{}\'>'.format(self.path)
-
-    def load(self, reqfile):
-        if not os.path.exists(reqfile):
-            raise ValueError('The given requirements file does not exist.')
-
-        finder = PackageFinder([], [], session=requests)
-        for requirement in parse_requirements(reqfile, finder=finder, session=requests):
-            if requirement.req:
-                if not getattr(requirement.req, 'name', None):
-                    # Prior to pip 8.1.2 the attribute `name` did not exist.
-                    requirement.req.name = requirement.req.project_name
-                self.requirements.append(requirement.req)
-
-
-def grep(reqfile, packages, silent=False):
-    try:
-        r = Requirements(reqfile)
-    except ValueError:
-        if not silent:
-            print('There was a problem loading the given requirement file.')
-        exit(os.EX_NOINPUT)
-
-    for req in r.requirements:
-        if req.name in packages:
-            if not silent:
-                print('Package {} found!'.format(req.name))
-            exit(0)
-
-    if not silent:
-        print('Not found.')
-
-    exit(1)
-
-
-def main():
-    args = docopt(__doc__, version='pip-grep')
-
-    kwargs = {'reqfile': args['<reqfile>'], 'packages': args['<package>'], 'silent': args['-s']}
-
-    grep(**kwargs)
-
-
-if __name__ == '__main__':
-    try:
-        main()
-    except Exception:
-        sys.exit(1)
-