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/openwrt/readme.md

@@ -87,13 +87,9 @@ Please check if the `STAGING_DIR` is configured correctly and that the toolchain
 
 ```
 $ ./tools/build.py --toolchain cmake/toolchain_openwrt_mips.cmake \
-                   --jerry-libc OFF \
                    --lto OFF
 ```
 
-Currenlty the JerryScript libc does not supports the mips platform, that's why the `--jerry-libc OFF` argument
-is passed during build.
-
 ### 2. Copy the binary
 
 After a successful build the `build/bin/jerry` binary file can be copied to the target device.