[Transform] Remove redundant declaration PollyAllowFullFunction (NFC)

The variable is declared in ScopDetection.h, which ScopInliner.cpp
includes.

Identified by readability-redundant-declaration.

Author: kazutakahirata

polly/lib/Transform/ScopInliner.cpp

@@ -26,8 +26,6 @@
 using namespace llvm;
 using namespace polly;
 
-extern bool polly::PollyAllowFullFunction;
-
 namespace {
 class ScopInliner : public CallGraphSCCPass {
   using llvm::Pass::doInitialization;