resize2fs: remove unused variable from adjust_superblock()

resize2fs: remove unused variable from adjust_superblock()

In adjust_superblock(), the 'group_block' variable is declared and set,
but it is never actually used.  Remove it.

This addresses the following compiler warning with clang -Wall:

        blk64_t         group_block;
                        ^

resize2fs.c:1119:11: warning: variable 'group_block' set but not used [-Wunused-but-set-variable]
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>