[Builtins] Remove stale comment. NFC

The header name was made a separate string more than 12.5 years ago.
I think it was part of the attribute string for less than a week.

clang/include/clang/Basic/Builtins.def

@@ -80,9 +80,7 @@
 // builtin even if type doesn't match signature, and don't warn if we
 // can't be sure the type is right
 // F -> this is a libc/libm function with a '__builtin_' prefix added.
-// f -> this is a libc/libm function without the '__builtin_' prefix. It can
-// be followed by ':headername:' to state which header this function
-// comes from.
+// f -> this is a libc/libm function without the '__builtin_' prefix.
 // h -> this function requires a specific header or an explicit declaration.
 // i -> this is a runtime library implemented function without the
 // '__builtin_' prefix. It will be implemented in compiler-rt or libgcc.