Remove redundant Mercurial install step (#1111)

Mercurial is installed in the stack image for all stacks, so the
pip install of packages from Mercurial VCS URLs works without the
need for the buildpack to install it itself.

See:
https://github.com/heroku/stack-images/pull/141
https://github.com/heroku/stack-images/search?q=mercurial

Closes @W-7906950@.

test/fixtures/requirements-mercurial/requirements.txt

@@ -0,0 +1 @@
+hg+https://www.mercurial-scm.org/repo/python-hglib/#egg=python-hglib

test/run-deps

@@ -74,6 +74,12 @@ testPylibmc() {
   assertCapturedSuccess
 }
 
+testMercurial() {
+  compile "requirements-mercurial"
+  assertCaptured "Cloning hg"
+  assertCapturedSuccess
+}
+
 pushd $(dirname 0) >/dev/null
 popd >/dev/null