Remove !USE_RVARGC code

[Feature #19579]

The Variable Width Allocation feature was turned on by default in Ruby
3.2. Since then, we haven't received bug reports or backports to the
non-Variable Width Allocation code paths, so we assume that nobody is
using it. We also don't plan on maintaining the non-Variable Width
Allocation code, so we are going to remove it.

This reverts commit c365c5921ea26e31c03a85b01ff4c04629abfc10.

This reverts commit d4a95428bb244ca8c4a97ad50f3837f191f1f0c3.

This commit adds support for embedded strings with variable capacity and
uses Variable Width Allocation to allocate strings.

It is definitely desirable to document them, but we need more human
resources.  Tentatively disable documenting them. [ci skip]

Truly editorial fix for comments.  This works better with Emacs'
set-justification-full function. [ci skip]