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

targets/esp8266/Makefile.esp8266

@@ -50,7 +50,6 @@ jerry:
 	 -DCMAKE_C_COMPILER_WORKS=TRUE \
 	 -DENABLE_LTO=OFF \
 	 -DENABLE_ALL_IN_ONE=ON \
-	 -DJERRY_LIBC=OFF \
 	 -DJERRY_CMDLINE=OFF \
 	 -DEXTERNAL_COMPILE_FLAGS="$(ESP_CFLAGS)" \
 	 -DMEM_HEAP_SIZE_KB=$(JERRYHEAP)