Fix BANK(@) discrepancy within LOAD blocks (#2149)

RGBASM and RGBLINK disagreed on which section to consider `@` to be in;
we have decided that the latter is correct.

The `assume` change is known to be suspicious; for more discussion, see
https://github.com/gbdev/rgbds/pull/2149#discussion_r4106902830

include/asm/section.hpp

@@ -5,7 +5,6 @@
 
 #include <deque>
 #include <memory>
-#include <optional>
 #include <stddef.h>
 #include <stdint.h>
 #include <string>
@@ -78,7 +77,6 @@ void sect_CheckLoadClosed();
 Section *sect_GetSymbolSection();
 uint32_t sect_GetSymbolOffset();
 uint32_t sect_GetOutputOffset();
-std::optional<uint32_t> sect_GetOutputBank();
 
 Patch *sect_AddOutputPatch();