Remove description for removed warning 51

Author: mediremi

compiler/ext/warnings.ml

@@ -638,7 +638,6 @@ let descriptions =
     (48, "Implicit elimination of optional arguments.");
     (49, "Absent cmi file when looking up module alias.");
     (50, "Unexpected documentation comment.");
-    (51, "Warning on non-tail calls if @tailcall present.");
     (52, "Fragile constant pattern.");
     (53, "Attribute cannot appear in this context");
     (54, "Attribute used more than once on an expression");