misc: Removed unneeded Doxygen pages

misc: Removed unneeded Doxygen pages

These removed doxygen files have already been migrated to the gem5
website.

inside-minor.doxygen:
www.gem5.org/documentation/general_docs/cpu_models/minor_cpu

memory_system.doxygen:
www.gem5.org/documentation/general_docs/memory_system/gem5_memory_system

power_thermal_model.doxygen:
www.gem5.org/documentation/general_docs/thermal_model

Issue-on: https://gem5.atlassian.net/browse/GEM5-229
Change-Id: Ib36c364def2dae06a0efbedd3d398763ae7d4e21
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27487
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>