Pure storage removal: Remove documentation

Pure storage removal: Remove documentation

As pure storage is now gone, it no longer needs to be documented.

* doc/lispref/elisp.texi (Top):
* doc/lispref/internals.texi (GNU Emacs Internals): Remove "Pure
Storage" section.
(Building Emacs, Garbage Collection, Writing Emacs Primitives):
* doc/lispref/symbols.texi (Standard Properties): Remove references to
pure storage.
* src/alloc.c (Fgarbage_collect): Remove docstring text referring to
pure storage.

(cherry picked from commit c9ab3258760c5ef2baf3ecf2a2a0051fc3fb4612)