cmake: Remove core_sanitizer_{cxx,linker}_flags helper variables

This change make the code more concise and minimizes the diff in the
subsequent commit.

src/CMakeLists.txt

@@ -51,12 +51,10 @@ set(SECP256K1_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
 include(GetTargetInterface)
 # -fsanitize and related flags apply to both C++ and C,
 # so we can pass them down to libsecp256k1 as CFLAGS and LDFLAGS.
-get_target_interface(core_sanitizer_cxx_flags "" sanitize_interface COMPILE_OPTIONS)
-set(SECP256K1_APPEND_CFLAGS ${core_sanitizer_cxx_flags} CACHE STRING "" FORCE)
-unset(core_sanitizer_cxx_flags)
-get_target_interface(core_sanitizer_linker_flags "" sanitize_interface LINK_OPTIONS)
-set(SECP256K1_APPEND_LDFLAGS ${core_sanitizer_linker_flags} CACHE STRING "" FORCE)
-unset(core_sanitizer_linker_flags)
+get_target_interface(SECP256K1_APPEND_CFLAGS "" sanitize_interface COMPILE_OPTIONS)
+set(SECP256K1_APPEND_CFLAGS ${SECP256K1_APPEND_CFLAGS} CACHE STRING "" FORCE)
+get_target_interface(SECP256K1_APPEND_LDFLAGS "" sanitize_interface LINK_OPTIONS)
+set(SECP256K1_APPEND_LDFLAGS ${SECP256K1_APPEND_LDFLAGS} CACHE STRING "" FORCE)
 # We want to build libsecp256k1 with the most tested RelWithDebInfo configuration.
 enable_language(C)
 foreach(config IN LISTS CMAKE_BUILD_TYPE CMAKE_CONFIGURATION_TYPES)