win32/Makefile.am: Remove generated files on 'make maintainer-clean'

Hi,

From the commit message, as a continuation of the suggestions given in MR !3 (closed):

config.h.win32, config-msvc.mak and Makefile are all generated from template files during the configure stage, so no need to keep them after performing a maintainer-clean.

I am not sure about how git.mk comes into action for generating .gitignore files, so I did not have the chance to take a go in trying to have a .gitignore file that contains the 3 files that are meant to be removed on make maintainer-clean.

With blessings, thank you!