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/nuttx-stm32f4/Makefile.travis

@@ -51,7 +51,7 @@ install: install-apt-get-deps install-noapt
 
 # Build JerryScript.
 script-build-jerryscript:
-	tools/build.py --clean --toolchain cmake/toolchain_mcu_stm32f4.cmake --profile=es2015-subset --jerry-cmdline OFF --jerry-libc OFF --lto OFF --jerry-libm ON --all-in-one ON --mem-heap 70 --compile-flag='--sysroot=../nuttx'
+	tools/build.py --clean --toolchain cmake/toolchain_mcu_stm32f4.cmake --profile=es2015-subset --jerry-cmdline OFF --lto OFF --jerry-libm ON --all-in-one ON --mem-heap 70 --compile-flag='--sysroot=../nuttx'
 
 # Link in the NuttX JerryScript target directory under the NuttX apps tree.
 script-add-jerryscript-app:

targets/nuttx-stm32f4/README.md

@@ -41,7 +41,6 @@ jerryscript/tools/build.py \
     --clean \
     --lto=OFF \
     --jerry-cmdline=OFF \
-    --jerry-libc=OFF \
     --jerry-libm=ON \
     --all-in-one=ON \
     --mem-heap=70 \