m68k: Drop -fno-strength-reduce from KBUILD_CFLAGS

This workaround became redundant either when the driver in question was
removed (in Linux v2.6.23) or when the compiler flag became a no-op
(in GCC v4.2). Linux has required GCC v4.6 or later since v4.19.

Link: https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=efa1cdf01850b28c2f6f7035ebd4420259494615


References: commit 565bae6a ("[SCSI] 53c7xx: kill driver")
References: commit cafa0010 ("Raise the minimum required gcc version to 4.6")
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Link: https://lore.kernel.org/r/baa95d7235921dff23bed6320518f3fa90396603.1612681353.git.fthain@telegraphics.com.au


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>