[th] Update countries.txt

[bact]

• Remove "ประเทศ" ("country") and "รัฐ" ("state") prefixes from names
• While in a lot of names, the prefix is valid, it cause confusion in others.
• "herzegovina", for example, is not a country but it has the prefix "country"
• In other cases, the country status is in dispute. For example, "abkhazia".
• To keep all the names consistent (within Thai language and with other langs) and avoid unnecessary specificity, the status prefixes should be removed