Revert "Fix up links"

This reverts commit 7f1d1c6d9a7be5e427bace30e740b16b25f25c92.

The original commit was created because mdBook and rustdoc had
different generation algorithms for header links; now with
https://github.com/rust-lang/rust/pull/39966 , the algorithms
are the same. So let's undo this change.

... when I came across this problem, I said "eh, this isn't fun,
but it doesn't take that long." I probably should have just actually
taken the time to fix upstream, given that they were amenable. Oh
well!

src/lifetime-mismatch.md

@@ -78,4 +78,4 @@ TODO: other common problems? SEME regions stuff, mostly?
 
 
 
-[ex2]: lifetimes.html#Example%3A%20aliasing%20a%20mutable%20reference
+[ex2]: lifetimes.html#example-aliasing-a-mutable-reference

src/repr-rust.md

@@ -151,4 +151,4 @@ use fairly elaborate algorithms to cache bits throughout nested types with
 special constrained representations. As such it is *especially* desirable that
 we leave enum layout unspecified today.
 
-[dst]: exotic-sizes.html#Dynamically%20Sized%20Types%20(DSTs)
+[dst]: exotic-sizes.html#dynamically-sized-types-dsts