Revert "[flang] Fix seg fault CodeGenAction::executeAction() (llvm#78269)" (llvm#78667)

This reverts commit 99cae9a.

Temporarily until I reproduce and fix a linker issue:
```
FAILED: tools/flang/unittests/Frontend/FlangFrontendTests
...
/usr/bin/ld: tools/flang/unittests/Frontend/CMakeFiles/FlangFrontendTests.dir/CodeGenActionTest.cpp.o: undefined reference to symbol '_ZN4llvm11LLVMContextC1Ev'
/usr/bin/ld: /work1/omp-nightly/build/git/trunk18.0/build/llvm-project/lib/libLLVMCore.so.18git: error adding symbols: DSO missing from command line
```

Author: ergawy

flang/lib/Frontend/FrontendActions.cpp

@@ -1202,11 +1202,6 @@ void CodeGenAction::executeAction() {
   if (!llvmModule)
     generateLLVMIR();
 
-  // If generating the LLVM module failed, abort! No need for further error
-  // reporting since generateLLVMIR() does this already.
-  if (!llvmModule)
-    return;
-
   // Set the triple based on the targetmachine (this comes compiler invocation
   // and the command-line target option if specified, or the default if not
   // given on the command-line).

flang/unittests/Frontend/CMakeLists.txt

@@ -4,7 +4,6 @@ set(LLVM_LINK_COMPONENTS
 )
 
 add_flang_unittest(FlangFrontendTests
-  CodeGenActionTest.cpp
   CompilerInstanceTest.cpp
   FrontendActionTest.cpp
 )