memblock: get rid of a :c:type leftover

chanseset b3a7bb1851c8 ("docs: get rid of :c:type explicit declarations for structs")
removed several :c:type: markups, except by one.

Now, Sphinx 3.x complains about it:

	.../Documentation/core-api/boot-time-mm:26: ../mm/memblock.c:51: WARNING: Unparseable C cross-reference: 'struct\nmemblock_type'
	Invalid C declaration: Expected identifier in nested name, got keyword: struct [error at 6]
	  struct
	memblock_type
	  ------^

As, on Sphinx 3.x, the right markup is c:struct:`foo`.

So, let's remove it, relying on automarkup.py to convert it.

Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>