Remove mkdep.awk

Remove mkdep.awk

The `mkdep.awk` file was part of the previous *nix build system and was
used to create a .deps file with a list of dependencies that could be
processed by Automake further on.

Newer build system was done via 9d9d39a0de3bec962c343051011f5a2ed7d7b242
and outdated files removed via 22815419f8c5da902971d3aa12f2cbfcc3b41aff
so the current file in the PHP source code is not used anymore.

Additionally, the *.slo files were processed by this file. The *.slo
files also used to be generated by older libtool so today, these don't
get generated anymore.