lib/bootmem.c: Remove unused setting of variable "begin"

The variable "begin" is extracted from the structure, but 4 lines below
it's overwritten with "end - size". This causes a static build scan error
that should be fixed. Remove the initial assignment of variable "begin".

BUG=b:112253891
TEST=Build and boot grunt.

Change-Id: I0a265747e61289f045c5cac09e40478bd31e16fc
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/27886
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/bootmem.c

@@ -278,7 +278,6 @@ void *bootmem_allocate_buffer(size_t size)
 		return NULL;
 
 	/* region now points to the highest usable region for the given size. */
-	begin = range_entry_base(region);
 	end = range_entry_end(region);
 	if (end > max_addr)
 		end = max_addr;