[MC] Remove assert to work around BOLT

BOLT used a dummy `MCAsmLayout` to call `getSymbolOffset`, which is
technically not supported. There is some discussion in
https://reviews.llvm.org/D154604 that this is not ideal.
For now, remove the assert.

The assert was added by bbb5036.

Author: MaskRay

llvm/lib/MC/MCAssembler.cpp

@@ -538,7 +538,6 @@ bool MCAssembler::getSymbolOffset(const MCSymbol &S, uint64_t &Val) const {
 }
 
 uint64_t MCAssembler::getSymbolOffset(const MCSymbol &S) const {
-  assert(HasLayout);
   uint64_t Val;
   getSymbolOffsetImpl(*this, S, true, Val);
   return Val;