[lldb] build: cleanup extraneous include paths (#117615)

Clean up some unnecessary include paths. The use of `LibXml2::LibXml2`
with `target_link_libraries` on `libLLDBHost` ensures that the header
search path is properly propagated.

Author: compnerd

lldb/cmake/modules/LLDBConfig.cmake

@@ -239,10 +239,6 @@ if (LLDB_ENABLE_LZMA)
   include_directories(${LIBLZMA_INCLUDE_DIRS})
 endif()
 
-if (LLDB_ENABLE_LIBXML2)
-  include_directories(${LIBXML2_INCLUDE_DIR})
-endif()
-
 include_directories(BEFORE
   ${CMAKE_CURRENT_BINARY_DIR}/include
   ${CMAKE_CURRENT_SOURCE_DIR}/include
@@ -283,7 +279,6 @@ if (APPLE)
   find_library(FOUNDATION_LIBRARY Foundation)
   find_library(CORE_FOUNDATION_LIBRARY CoreFoundation)
   find_library(SECURITY_LIBRARY Security)
-  include_directories(${LIBXML2_INCLUDE_DIR})
 endif()
 
 if( WIN32 AND NOT CYGWIN )