emitters: Remove useless symbGetLen() <= 0 checks

There are no variables with <= 0 length ever created, at least not anymore.
symb: Remove symbIsLiteralConst() (unused)
symb: Solve out FB_SYMBATTRIB_LITCONST
ASM backend: Remove useless check for literal UDTs

There are no struct literals ever created.
emitters: Remove useless symbGetVarIsAllocated() checks

None of the added assert()s trigger when compiling the test suite, so
I guess that none of that is needed (anymore). It makes sense too because
as far as I can tell the backends only try to emit each variable once.
symb: Remove FB_SYMBSTATS_VARALLOCATED (no longer used)