ubifs: Fix some kernel-doc warnings in gc.c

Fixes the following W=1 kernel build warning(s):

fs/ubifs/gc.c:70: warning: Excess function parameter 'buf' description in 'switch_gc_head'
fs/ubifs/gc.c:70: warning: Excess function parameter 'len' description in 'switch_gc_head'
fs/ubifs/gc.c:70: warning: Excess function parameter 'lnum' description in 'switch_gc_head'
fs/ubifs/gc.c:70: warning: Excess function parameter 'offs' description in 'switch_gc_head'

They're not in use. Remove them.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>