Drop UTF-8 string functions

We don't use them internally and they aren't exactly relevant to the
library, so let's remove them.

Fixes Librem5/libhandy#207.
11 jobs for drop-string-funcs in 15 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #251499
librem5
api-visibility

00:02:48

passed #251501
librem5
build-debian-clang

00:04:09

passed #251500
librem5
build-debian-gcc

00:03:19

passed #251502
librem5
build-fedora-gcc

00:04:00

 
  Test+Docs
passed #251503
librem5
build-gtkdoc

00:03:00

passed #251504
librem5
unit-test

00:03:00

41.7%
 
  Package
passed #251505
librem5
package-debian-buster

00:02:36

passed #251506
librem5:arm64
package-debian-buster:arm64

00:06:01

passed #251507
flatpak
package-flatpak

00:01:40

 
  Test Package
passed #251508
librem5
autopkgtest-debian-buster-package

00:02:32

passed #251509
librem5
lintian-debian-buster-package

00:01:01