puppet: Remove custom vm.dirty_ratio and vm.dirty_background_ratio.

These values differed between the primary and secondary database
hosts, for unclear reasons.  The differences date back to their
introduction in 387f63deaa.  As the comment in the replica
confguration notes, settings of `vm.dirty_ratio = 10` and
`vm.dirty_background_ratio = 5` matched the kernel defaults for
"newer" kernels; however, kernel 2.6.30 bumped those to 20 and 10,
respectively[1], as a fix for underlying logic now being more correct.

Remove these overrides; they should at very least be consistent across
roles, and the previous values look to be an attempt to tune for a
very much older version of the Linux kernel, which was using an
different, buggier, algorithm under the hood.

[1] 1b5e62b42b

puppet/zulip_ops/files/postgresql/40-postgresql.conf

@@ -0,0 +1,3 @@
+# Virtual memory settings
+vm.swappiness = 0
+vm.overcommit_memory = 2

puppet/zulip_ops/files/postgresql/40-postgresql.conf.master

@@ -1,9 +0,0 @@
-# Virtual memory settings
-vm.swappiness = 0
-vm.overcommit_memory = 2
-
-# We don't want these numbers to be too large or else performance
-# will be too spiky.  These seem to give a good balance between low
-# numbers and sequential write performance
-vm.dirty_ratio = 5
-vm.dirty_background_ratio = 2

puppet/zulip_ops/files/postgresql/40-postgresql.conf.slave

@@ -1,7 +0,0 @@
-# Virtual memory settings
-vm.swappiness = 0
-vm.overcommit_memory = 2
-
-# These are the defaults on newer kernels
-vm.dirty_ratio = 10
-vm.dirty_background_ratio = 5