internal/memoize: remove unused Store.generations map
internal/memoize: remove unused Store.generations map

This change removes an unused map, renames Store.mu, and
add minor commentary.

Change-Id: I2f064ff0daf87e0f73930bc980760a453d18e70a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/414494
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools