xfs: stop CONFIG_XFS_DEBUG from changing compiler flags

I ran into a linker warning in XFS that originates from a mismatch
between libelf, binutils and objtool when certain files in the kernel
are built with "gcc -g":

x86_64-linux-ld: fs/xfs/xfs_trace.o: unable to initialize decompress status for section .debug_info

After some discussion, nobody could identify why xfs sets this flag
here. CONFIG_XFS_DEBUG used to enable lots of unrelated settings, but
now its main purpose is to enable extra consistency checks and assertions
that are unrelated to the debug info.

Remove the Makefile logic to set the flag here. If anyone relies
on the debug info, this can simply be enabled again with the global
CONFIG_DEBUG_INFO option.

Dave Chinner writes:

I'm pretty sure it was needed for the original kgdb integration back
in the early 2000s. That was when SGI used to patch their XFS dev
tree with kgdb and debug symbols were needed by the custom kgdb
modules that were ported across from the Irix kernel debugger.

ISTR that the early kcrash kernel dump analysis tools (again,
originated from the Irix "icrash" kernel dump tools) had custom XFS
debug scripts that needed also the debug info to work correctly...

Which is a long way of saying "we don't need it anymore" instead of
"nobody knows why it was set"... :)
Suggested-by: Christoph Hellwig <hch@infradead.org>
Link: https://lore.kernel.org/lkml/20200409074130.GD21033@infradead.org/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/Makefile

@@ -7,8 +7,6 @@
 ccflags-y += -I $(srctree)/$(src)		# needed for trace events
 ccflags-y += -I $(srctree)/$(src)/libxfs
 
-ccflags-$(CONFIG_XFS_DEBUG) += -g
-
 obj-$(CONFIG_XFS_FS)		+= xfs.o
 
 # this one should be compiled first, as the tracing macros can easily blow up