Revert "Allow 2x1 calls in the "strict_standard_callsign_re" regular expression."

This reverts commit 575a5c5a7a.

Radio.cpp

@@ -22,7 +22,7 @@ namespace Radio
     QRegularExpression valid_callsign_regexp {R"(\d[[:alpha:]]|[[:alpha:]]\d)"};
 
     // standard callsign
-    QRegularExpression strict_standard_callsign_re {R"(^([A-Z]{1,2}[0-9]?|[0-9A-Z][A-Z])[0-9][A-Z]{0,3}$)"};
+    QRegularExpression strict_standard_callsign_re {R"(^([A-Z][0-9]?|[0-9A-Z][A-Z])[0-9][A-Z]{0,3}$)"};
 
     // suffixes that are often used and should not be interpreted as a
     // DXCC Entity prefix used as a suffix