[llvm] remove unusable llvm.spec.in (llvm#96825)

The `llvm.spec.in` is turned into `llvm.spec` through cmake. The spec
file's `%build` section runs `./configure` which has been deprecated
since 2016 (See e49730d).

llvm/CMakeLists.txt

@@ -1135,25 +1135,6 @@ configure_file(
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Config/abi-breaking.h.cmake
   ${LLVM_INCLUDE_DIR}/llvm/Config/abi-breaking.h)
 
-# Add target for generating source rpm package.
-set(LLVM_SRPM_USER_BINARY_SPECFILE ${CMAKE_CURRENT_SOURCE_DIR}/llvm.spec.in
-    CACHE FILEPATH ".spec file to use for srpm generation")
-set(LLVM_SRPM_BINARY_SPECFILE ${CMAKE_CURRENT_BINARY_DIR}/llvm.spec)
-set(LLVM_SRPM_DIR "${CMAKE_CURRENT_BINARY_DIR}/srpm")
-
-get_source_info(${CMAKE_CURRENT_SOURCE_DIR} revision repository)
-string(LENGTH "${revision}" revision_length)
-set(LLVM_RPM_SPEC_REVISION "${revision}")
-
-configure_file(
-  ${LLVM_SRPM_USER_BINARY_SPECFILE}
-  ${LLVM_SRPM_BINARY_SPECFILE} @ONLY)
-
-add_custom_target(srpm
-  COMMAND cpack -G TGZ --config CPackSourceConfig.cmake -B ${LLVM_SRPM_DIR}/SOURCES
-  COMMAND rpmbuild -bs --define '_topdir ${LLVM_SRPM_DIR}' ${LLVM_SRPM_BINARY_SPECFILE})
-set_target_properties(srpm PROPERTIES FOLDER "LLVM/Misc")
-
 if(APPLE AND DARWIN_LTO_LIBRARY)
   set(CMAKE_EXE_LINKER_FLAGS
     "${CMAKE_EXE_LINKER_FLAGS} -Wl,-lto_library -Wl,${DARWIN_LTO_LIBRARY}")