[Sema] Remove unused variable 'FromExtType' [NFC]
Last use of the variable was removed in a29afb754fb4
[HLSL] Allow truncation to scalar (#104844)
gcc warned about this:
../../clang/lib/Sema/SemaOverload.cpp:2070:15: warning: unused variable 'FromExtType' [-Wunused-variable]
2070 | if (auto *FromExtType = FromType->getAs<ExtVectorType>()) {
| ^~~~~~~~~~~
