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

jerry-main/CMakeLists.txt

@@ -54,10 +54,6 @@ macro(jerry_create_executable JERRY_NAME)
   install(TARGETS ${JERRY_NAME} DESTINATION bin)
 endmacro()
 
-if(JERRY_LIBC AND FEATURE_EXTERNAL_CONTEXT)
-  MESSAGE(FATAL_ERROR "This configuration is not supported for jerry-main. Please build against your system libc to enable the external context.")
-endif()
-
 # Jerry standalones
 if(JERRY_CMDLINE)
   jerry_create_executable("jerry" "main-unix.c" "cli.c")