selfhost: drop dead 'import mem' (#16 PREP-deadimport)

mem.ww was retired in Phase-0; these two imports survived only because
the driver silently skipped unresolvable imports. Provably dead: no mem
module exists and both units build identically without them. Removal
precedes the loud driver, which would otherwise fatal on them.

selfhost/cmd/w6a/asm.ww

@@ -13,7 +13,6 @@ package w6a;
 
 import os;
 import rt;
-import mem;
 import opcodes;
 import strings;
 

selfhost/cmd/w6a/main.combined.ww

@@ -3684,7 +3684,6 @@ package w6a;
 
 import os;
 import rt;
-import mem;
 import opcodes;
 import strings;
 

selfhost/test/uses.ww

@@ -8,7 +8,6 @@
 package test;
 
 import os;
-import mem;
 import fmt;
 
 def MAX_LINE: i32 = 4096;