docs: remove misleading link from Don't over-optimize section

The link to github.com/petkaantonov/bluebird/wiki/Optimization-killers
was misleading — it pointed to an outdated Bluebird Promise library
wiki page about V8 optimization killers that is no longer maintained
or accurate for modern JavaScript engines.

Replace with a general recommendation to use browser developer tools
and performance profiling to identify real bottlenecks, which is more
actionable and up-to-date advice.

Closes ryanmcdermott#719

Author: Eduardo Bar

README.md

@@ -977,10 +977,9 @@ function combine(val1, val2) {
 ### Don't over-optimize
 
 Modern browsers do a lot of optimization under-the-hood at runtime. A lot of
-times, if you are optimizing then you are just wasting your time. [There are good
-resources](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers)
-for seeing where optimization is lacking. Target those in the meantime, until
-they are fixed if they can be.
+times, if you are optimizing then you are just wasting your time. Use browser
+developer tools and performance profiling to identify real bottlenecks before
+optimizing. Target those in the meantime, until they are fixed if they can be.
 
 **Bad:**