kconfig: remove 'config*' pattern from .gitignnore

I could not figure out why this pattern should be ignored.
Checking commit 1e65174a

 ("Add some basic .gitignore files")
did not help.

Let's remove this pattern, then see if it is really needed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>