[Truffle] Untagged some passing String specs.

Author: nirvdrum

spec/truffle/tags/core/string/append_tags.txt

@@ -1,5 +1,3 @@
-fails:String#<< converts the given argument to a String using to_str
-fails:String#<< raises a RuntimeError when self is frozen
 fails:String#<< taints self if other is tainted
 fails:String#<< untrusts self if other is untrusted
 fails:String#<< with Integer concatencates the argument interpreted as a codepoint

spec/truffle/tags/core/string/gsub_tags.txt

@@ -11,7 +11,6 @@ fails:String#gsub with pattern and replacement taints the result if the original
 fails:String#gsub with pattern and replacement untrusts the result if the original string or replacement is untrusted
 fails:String#gsub with pattern and replacement tries to convert pattern to a string using to_str
 fails:String#gsub with pattern and replacement tries to convert replacement to a string using to_str
-fails:String#gsub with pattern and replacement returns subclass instances when called on a subclass
 fails:String#gsub with pattern and replacement sets $~ to MatchData of last match and nil when there's none
 fails:String#gsub with pattern and Hash returns a copy of self with all occurrences of pattern replaced with the value of the corresponding hash key
 fails:String#gsub with pattern and Hash ignores keys that don't correspond to matches