Don't set the INITRD_COMPRESS environment variable automatically

Commit 1bf49dd4

 ("./Makefile: export initial ramdisk compression
config option") started setting the INITRD_COMPRESS environment variable
depending on which decompression models the kernel had available.

That is completely broken.

For example, we by default have CONFIG_RD_LZ4 enabled, and are able to
decompress such an initrd, but the user tools to *create* such an initrd
may not be availble.  So trying to tell dracut to generate an
lz4-compressed image just because we can decode such an image is
completely inappropriate.

Cc: J P <ppandit@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>