kbuild: modinst: remove the multithread option from zstd compression

Parallel execution is supported by GNU Make:

$ make -j<N> modules_install

It is questionable to enable multithreading within each zstd process
by default.

If you still want to do it, you can use the environment variable:

$ ZSTD_NBTHREADS=<N> make modules_install

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>
Tested-by: Sedat Dilek <[e-mail]>

scripts/Makefile.modinst

···
 quiet_cmd_xz = XZ      $@
       cmd_xz = $(XZ) --check=crc32 --lzma2=dict=1MiB -f $<
 quiet_cmd_zstd = ZSTD    $@
-      cmd_zstd = $(ZSTD) -T0 --rm -f -q $<
+      cmd_zstd = $(ZSTD) --rm -f -q $<
 
 $(dst)/%.ko.gz: $(dst)/%.ko FORCE
 	$(call cmd,gzip)