Remove chars_equals_insensitive and chars_less_insensitive

These aren't used anymore as of #9332 (save one place, which has been inlined here), and they're not good to rely on, since they don't handle utf8 properly.

src/serialization/string_utils.cpp

@@ -663,7 +663,7 @@ bool word_completion(std::string& text, std::vector<std::string>& wordlist) {
 	{
 		if (word->size() < semiword.size()
 		|| !std::equal(semiword.begin(), semiword.end(), word->begin(),
-				utils::chars_equal_insensitive))
+			[](char a, char b) { return tolower(a) == tolower(b); })) // TODO: is this the right approach?
 		{
 			continue;
 		}

src/utils/general.hpp

@@ -20,9 +20,6 @@
 
 namespace utils
 {
-inline bool chars_equal_insensitive(char a, char b) { return tolower(a) == tolower(b); }
-inline bool chars_less_insensitive(char a, char b) { return tolower(a) < tolower(b); }
-
 /**
  * Equivalent to as @c std::is_same_v except both types are passed through std::decay first.
  *