config: remove m4/.gitignore now that toplevel one handles it all The -Im4 in Makefile.am will have to wait until the day there is an m4 macro checked-in git in the m4 directory. This does not prevent libtool to install its macros in m4. Acked-by: Daniel Stone <daniel@fooishbar.org> Signed-off-by: Gaetan Nadon <memsize@videotron.ca>