Remove Mercurial VCS test (#1362)

Since:
- The test is currently failing due to https://www.mercurial-scm.org being
  down, and there aren't many/any other decent testcase alternatives,
  since so few projects use Mercurial these days (and we also need a
  testcase that's quick to install and doesn't need additional system libs).
- All the test is really testing is that (a) the Mercurial binary is installed in
  the stack image, (b) that we haven't messed up the `PATH` passed to
  pip, (c) that Pip doesn't have a regression with Mercurial VCS support.
  However (a) isn't really a concern of this buildpack, (b) is still tested via
  the Git testcase, and (c) is unlikely/really an upstream concern (it's not
  viable/a good use of our time to try and replicate pip's test suite
  ourselves).

GUS-W-11716181.

Author: edmorley

spec/fixtures/requirements_git/requirements.txt

@@ -0,0 +1,2 @@
+# This relies upon the VCS binaries from the stack image.
+git+https://github.com/certifi/python-certifi

spec/fixtures/requirements_vcs/requirements.txt

@@ -81,8 +81,8 @@
     include_examples 'installs successfully using pip'
   end
 
-  context 'when requirements.txt contains Git/Mercurial requirements URLs' do
-    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_vcs') }
+  context 'when requirements.txt contains Git requirements URLs' do
+    let(:app) { Hatchet::Runner.new('spec/fixtures/requirements_git') }
 
     include_examples 'installs successfully using pip'
   end