Log Message:
pkgtools/pkglint4: remove more unreliable or outdated checks

CFLAGS and LDFLAGS had been checked for GCC-style flags only. This didn't
make sense since wrong flags would be detected on the exotic platforms
anyway.

The experimental --autofix has been removed.

The check for "# used by" comments in Makefile.common has been removed.
Especially the --autofix could damage files.

The warnings for ICON_THEMES have been removed since they appear in
groups of thousands, and the old pkglint cannot issue warnings "once per
file" simply.