kbuild: remove *.symversions left-over

Commit 5ce2176b81f7 ("genksyms: adjust the output format to modpost")
stopped generating *.symversions files.

Remove the left-over from the .gitignore file and the 'clean' rule.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
2 files changed