Remove stale and redundant src/.gitignore file

This commit removes completely the src/.gitignore file, given that the
precedent for ignoring artifacts within the `src` directory is to
add entries for them to the root .gitignore file.

Note also that the lone entry in src/.gitignore is stale anyway. As of
the switch to Autotools in 35b8af9, the build no longer build creates
artifacts in `src/test_bitcoin`. They are now written to
`src/test/test_bitcoin`, and this latter path is already ignored in the
root .gitignore file.
Merge pull request #3938

6540025 Remove stale and redundant src/.gitignore file (Chris Beams)