Remove jerry-libc (#2332)

Rationale:
- There is no port under targets/ that would use it. All of them
  turn it off when building.
- That's no surprise, as jerry-libc supports no barebone MCUs but
  posix targets with syscalls only. Actually, that's Linux only,
  because macOS builds have turned off the use of jerry-libc a
  while ago.
- And there is no point in maintaining a highly restricted set of
  libc functions: as soon as someone wants to use JerryScript in a
  scenario that needs more functions than jerry-main, they have to
  choose a different libc (most problably the compiler's default
  one).

I think that we should not keep supporting an otherwise unused
library for the purposes of jerry-main on arm/x86/x64-linux  only.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

tests/unit-libm/CMakeLists.txt

@@ -30,9 +30,6 @@ foreach(SOURCE_UNIT_TEST_MAIN ${SOURCE_UNIT_TEST_MAIN_MODULES})
   set_property(TARGET ${TARGET_NAME} PROPERTY RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/tests")
 
   target_link_libraries(${TARGET_NAME} jerry-libm)
-  if(JERRY_LIBC)
-    target_link_libraries(${TARGET_NAME} jerry-libc)
-  endif()
 
   add_dependencies(unittests-libm ${TARGET_NAME})
 endforeach()