METADATA.pb: Move sample_text and sample_glyphs to language textprotos

[felipesanches]

While updating the METADATA.pb schema description on FontBakery (fonttools/fontbakery#3443) and gf-tools (googlefonts/gf-docs#114), I noticed that some files such as https://github.com/google/fonts/blob/main/ofl/notosansoldpermic/METADATA.pb have sample_text and sample_glyphs entries and this seems redundant to the kind of content I see in some of the language textprotos available at https://github.com/google/fonts/blob/main/lang/languages/

On this specific case, though, https://github.com/google/fonts/blob/main/lang/languages/koi_Perm.textproto and https://github.com/google/fonts/blob/main/lang/languages/kv_Perm.textproto lack such sample entries.

The sample_text and sample_glyphs entries should (in my opinion) migrate to the language textproto files while the languages entries also present on the METADATA.pb file should be enough to cross-reference the lang database:

languages: "koi_Perm"  # Komi-Permyak, Old Permic
languages: "kv_Perm"  # Komi, Old Permic

[felipesanches]

Not sure if there are more of these commits, but at least 398a28a is one place where these entries were added recently.