Skip trying to link to libm explicitly

This doesn't seem to be necessary: it is automatically linked to,
at least on my machine. \*shrugs\*

src/CMakeLists.txt

@@ -118,9 +118,3 @@ else()
   target_include_directories(rgbgfx PRIVATE ${PNG_INCLUDE_DIRS})
   target_link_libraries(rgbgfx PRIVATE ${PNG_LIBRARIES})
 endif()
-
-include(CheckLibraryExists)
-check_library_exists("m" "sin" "" HAS_LIBM)
-if(HAS_LIBM)
-  target_link_libraries(rgbasm PRIVATE "m")
-endif()