Delete BigDiffTest

This test was originally added to make sure the `git` process does not hang
in case it writes a lot of output to the output streams.

To check this behavior, the test created 100.000 (!) files each a few KBs
large and then invoked the plugin, hoping that `git` would produce enough
output.
However, this puts a lot of pressure on the file system and disk due to
the large number of files.
For GitHub CI whose runners are probably using SSDs, this test alone
took more than 50% of the time of the complete Maven build. And when
running the test with an HDD the test could easily take minutes.

git-commit-id/git-commit-id-plugin-core#101
has added a dedicated and more reliable test which makes sure that the
process output is properly consumed, so there is probably no need for this
`BigDiffTest` anymore, especially due to the downsides mentioned above.

Author: Marcono1234