Remove useless member variables in PrintCompositor's internal structs

DocumentInfo and RequestInfo are two structs internal to
PrintCompositor. They each have a useless variable: one is never
referenced, while the other is incremented but never read. Remove both
of them. https://crrev.com/715570 added both of these variables but
never put them to effective use.

Change-Id: Iaad8d14b74930e4bf0b10f2eca92755f650b40e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5772574
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1339102}