Always enable native JSON support and remove Jansson references

Always enable native JSON support and remove Jansson references

* src/json.c (Fjson__available_p): Remove.
* lisp/subr.el (json-available-p): Always return t.
* admin/nt/dist-build/build-dep-zips.py:
* configure.ac:
* doc/lispref/text.texi (Parsing JSON):
* java/INSTALL:
* java/org/gnu/emacs/EmacsNative.java (EmacsNative):
* lisp/term/w32-win.el (dynamic-library-alist):
* m4/ndk-build.m4 (ndk_INIT):
* msdos/sed1v2.inp:
* nt/INSTALL:
* nt/INSTALL.W64:
* src/Makefile.in:
* src/emacs.c (main):
* src/lisp.h:
Remove JSON configuration options and references to it and Jansson from
documentation and build files.
* etc/NEWS: Announce.