[diff.cpp17.except] Fix Rationale to be a rationale.

"This was retained for one additional C++ standard to ease transition"
is not a rationale for removing the feature now.

Author: zygoloid

source/compatibility.tex

@@ -1958,8 +1958,7 @@
 \diffref{except.spec}
 \change Remove \tcode{throw()} exception specification.
 \rationale
-The empty dynamic exception specification was retained for one additional \Cpp{}
-Standard to ease the transition away from this feature.
+Removal of obsolete feature that has been replaced by \tcode{noexcept}.
 \effect
 A valid \CppXVII{} function declaration, member function declaration, function
 pointer declaration, or function reference declaration that uses \tcode{throw()}