Delete a disallowed template instantiation the proper way

C++11 allows you to do = delete to the declaration. But some version
of Clang on Mac doesn't like it. I'm guessing it's a compiler bug in
an older version of Clang used in the Qt CI system. I can't reproduce
the issue, so no further investigation was done.

So just remove the "inline" keyword, which is what ICC 14.0 complains
about:

MathExtras.h(270): error #864: extern inline function "clampTo(double, T, T) [with T=long long]" was referenced but not defined

Change-Id: I3887ccb527b5995ffa7ece612f9c7d5a80e71079
Reviewed-by: Alan Alpert <aalpert@blackberry.com>