Seems kenlm can be built with C++20, remove the CXX_STANDARD for kenlm

kenlm was compiled with C++11 while others using C++20 think the
std::string is violating odr.
Ref: https://bugs.gentoo.org/962546

Author: wengxt

src/libime/core/CMakeLists.txt

@@ -14,7 +14,6 @@ target_include_directories(kenlm PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE
 target_compile_definitions(kenlm PUBLIC -DKENLM_MAX_ORDER=3 PRIVATE -DNDEBUG)
 target_link_libraries(kenlm PUBLIC Boost::boost PkgConfig::ZSTD)
 set_target_properties(kenlm PROPERTIES
-  CXX_STANDARD 11
   POSITION_INDEPENDENT_CODE ON)
 
 if(UNIX)