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.

java/INSTALL

@@ -307,14 +307,6 @@ Many of these dependencies have been migrated over to the
 However, the old ``Android.mk'' Makefiles are still present in older
 branches, and can be easily adapted to newer versions.
 
-In addition, some Emacs dependencies provide `ndk-build' support
-themselves:
-
-  libjansson	- https://github.com/akheron/jansson
-    (You must add LOCAL_EXPORT_INCLUDES := $(LOCAL_C_INCLUDES) before
-     its Android.mk includes $(BUILD_SHARED_LIBRARY), then copy
-     android/jansson_config.h to android/jansson_private_config.h.)
-
 Emacs developers have ported the following dependencies to ARM Android
 systems: