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.