Remove LOAD FRAGMENT

It's very troublesome, with flaky semantics and a very restricted use space.

test/asm/load-fragment.asm

@@ -1,29 +1,2 @@
 SECTION "A", ROM0
-AData::
 LOAD FRAGMENT "RAM", WRAM0
-AMem::
-	db 0, 1, 2
-AMemEnd::
-ENDL
-ADataEnd::
-	dw AMem
-
-SECTION "B", ROM0
-BData::
-LOAD FRAGMENT "RAM", WRAM0
-BMem::
-	db 3, 4, 5, 6, 7
-BMemEnd::
-ENDL
-BDataEnd::
-	dw BMem
-
-SECTION "C", ROM0
-CData::
-LOAD FRAGMENT "RAM", WRAM0
-CMem::
-	db 8, 9
-CMemEnd::
-ENDL
-CDataEnd::
-	dw CMem

test/asm/load-fragment.err

@@ -0,0 +1,3 @@
+ERROR: load-fragment.asm(2):
+    `LOAD FRAGMENT` is not allowed
+error: Assembly aborted (1 error)!