Use CPAN distribution of the Git module

The Git module which comes in the git packages is now on CPAN[1],
courtesy of Mike South. So, this commit gets rid of all the code in
Makefile.PL that tries to find Git in nonstandard places. It also drops
the support for the GITPERLLIB environment variable.

Ref: [1] https://metacpan.org/release/Git
v1.1.0

  [Changes]

  - Use the CPAN distribution of the Git module instead of the Git.pm that
  comes with the git package. This makes Git::Hooks properly dependent of
  Git, which also means that the GITPERLLIB environment variable isn't
  needed anymore. (Many thanks to Mike South for taking care of the Git
  module maintenance.)