Revert "Use tmux as TERM if TERM is screen but TMUX env is set"

Forcing TERM to tmux can cause problems, because often "tmux" terminfo is unavailable, which is why tmux defaults to "screen". The tmux FAQ explicitly mentions that tmux uses "screen" by default because it is widely available. This fixes the crash issues in gomuks, described in this bug: tulir/gomuks#250

This reverts commit aef08b1.

tscreen.go

@@ -50,13 +50,9 @@ const (
 // $COLUMNS environment variables can be set to the actual window size,
 // otherwise defaults taken from the terminal database are used.
 func NewTerminfoScreen() (Screen, error) {
-	term := os.Getenv("TERM")
-	if len(term) >= 6 && term[:6] == "screen" && len(os.Getenv("TMUX")) > 0 {
-		term = "tmux"
-	}
-	ti, e := terminfo.LookupTerminfo(term)
+	ti, e := terminfo.LookupTerminfo(os.Getenv("TERM"))
 	if e != nil {
-		ti, e = loadDynamicTerminfo(term)
+		ti, e = loadDynamicTerminfo(os.Getenv("TERM"))
 		if e != nil {
 			return nil, e
 		}