x86/build: Remove redundant 'clean-files += capflags.c'

All the files added to 'targets' are cleaned. Adding the same file to both
'targets' and 'clean-files' is redundant.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20190625073311.18303-1-yamada.masahiro@socionext.com