[CMake] Don't pass -DBUILD_EXAMPLES to the build (#102838)

The only use in `opt.cpp` was removed in
d291f1fd094538af705541045c0d9c3ceb85e71d.

llvm/CMakeLists.txt

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 51f99cb..d681b1c 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -796,10 +796,6 @@
   "Build the LLVM example programs. If OFF, just generate build targets." OFF)
 option(LLVM_INCLUDE_EXAMPLES "Generate build targets for the LLVM examples" ON)
 
-if(LLVM_BUILD_EXAMPLES)
-  add_compile_definitions(BUILD_EXAMPLES)
-endif(LLVM_BUILD_EXAMPLES)
-
 option(LLVM_BUILD_TESTS
   "Build LLVM unit tests. If OFF, just generate build targets." OFF)
 option(LLVM_INCLUDE_TESTS "Generate build targets for the LLVM unit tests." ON)