StringUtil: Remove JoinStrings

With 12 uses of `JoinStrings` in the codebase vs 36 uses of `fmt::join`, fmtlib's range adapter for string concatenation with delimiters is clearly the preferred option.

Source/UnitTests/Common/StringUtilTest.cpp

@@ -7,15 +7,6 @@
 
 #include "Common/StringUtil.h"
 
-TEST(StringUtil, JoinStrings)
-{
-  EXPECT_EQ("", JoinStrings({}, ", "));
-  EXPECT_EQ("a", JoinStrings({"a"}, ","));
-  EXPECT_EQ("ab", JoinStrings({"a", "b"}, ""));
-  EXPECT_EQ("a, bb, c", JoinStrings({"a", "bb", "c"}, ", "));
-  EXPECT_EQ("???", JoinStrings({"?", "?"}, "?"));
-}
-
 TEST(StringUtil, StringPopBackIf)
 {
   std::string abc = "abc";