src: provide locale_charset() stub

Provide a stub function that lets us drop localcharset.c (and everything
else from libcharset) from the list of compiled dependencies.

Fixes #71.
deps: remove unused libiconv files

Delete unused libiconv files, like configure scripts, m4 macros, tests,
unused source files, etc.  Shrinks the download by about 13 MB.
package: 2.1.1 -> 2.1.2

Only notable change in this release is that the tarball is 13 MB
smaller now.