Remove ChunkAccess, ChunkCache

All of this has been dead code since the work to dispatch changes via
jobs; specifically, the last use of ChunkCache was removed in change
Ia9bf356584 (commit 008989a) and the last reference to ChunkAccess in
change Icef4da7e47 (commit 8a502ad).

testLoadByChangeIds() has to be rewritten to no longer depend on
testLoadChunk() (which previously inserted test data used by both
tests); as part of this, we can also simplify assertChangesEqual() now.

Bug: T292604
Change-Id: I7ad46c10be59a362245c8979f744462687bff3be

Author: lucaswerkmeister

.phpcs.xml

@@ -107,7 +107,6 @@
 	</rule>
 	<rule ref="MediaWiki.Usage.ForbiddenFunctions.assert">
 		<exclude-pattern>ItemChangeTest\.php</exclude-pattern>
-		<exclude-pattern>ChunkCache\.php</exclude-pattern>
 	</rule>
 	<rule ref="MediaWiki.Usage.ForbiddenFunctions.shell_exec">
 		<exclude-pattern>PidLock\.php</exclude-pattern>