[CMake] Remove {LIBCXX,LIBCXXABI,LIBUNWIND}_INSTALL_PREFIX

These variables were introduced during early work on the runtimes build
but were obsoleted by {LIBCXX,LIBCXXABI,LIBUNWIND}_INSTALL_LIBRARY_DIR.

Differential Revision: https://reviews.llvm.org/D99697

GitOrigin-RevId: 96d8c6b571e696f520a054820d13872cdc17c793
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 628f0aa..10c6b88 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -133,8 +133,6 @@
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${LIBUNWIND_LIBRARY_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${LIBUNWIND_LIBRARY_DIR})
 
-set(LIBUNWIND_INSTALL_PREFIX "" CACHE STRING "Define libunwind destination prefix.")
-
 set(LIBUNWIND_C_FLAGS "")
 set(LIBUNWIND_CXX_FLAGS "")
 set(LIBUNWIND_COMPILE_FLAGS "")
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index f341b21..03fd0fa 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -182,9 +182,9 @@
 
 if (LIBUNWIND_INSTALL_LIBRARY)
   install(TARGETS ${LIBUNWIND_INSTALL_TARGETS}
-    LIBRARY DESTINATION ${LIBUNWIND_INSTALL_PREFIX}${LIBUNWIND_INSTALL_LIBRARY_DIR} COMPONENT unwind
-    ARCHIVE DESTINATION ${LIBUNWIND_INSTALL_PREFIX}${LIBUNWIND_INSTALL_LIBRARY_DIR} COMPONENT unwind
-    RUNTIME DESTINATION ${LIBUNWIND_INSTALL_PREFIX}bin COMPONENT unwind)
+    LIBRARY DESTINATION ${LIBUNWIND_INSTALL_LIBRARY_DIR} COMPONENT unwind
+    ARCHIVE DESTINATION ${LIBUNWIND_INSTALL_LIBRARY_DIR} COMPONENT unwind
+    RUNTIME DESTINATION bin COMPONENT unwind)
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES AND LIBUNWIND_INSTALL_LIBRARY)