Remove cppcheck from AthenaExternals, master branch (2022.01.11.)

Removed cppcheck from AthenaExternals, because the current version of cppcheck is unfortunately not usable from CVMFS after installation. 😦 (See ATLINFR-2654.)

For now we will stick to using the installation provided by @desilva as part of ALRB, as discussed in a private e-mail thread.

[bash][atlas]:krasznaa > lsetup "cppcheck testing"
************************************************************************
Requested:  cppcheck ... 
 Setting up cppcheck 2.6-x86_64-centos7-gcc11 ... 
>>>>>>>>>>>>>>>>>>>>>>>>> Information for user <<<<<<<<<<<<<<<<<<<<<<<<<
 cppcheck:
   To use cppcheck you need to setup a release or a nightly first
************************************************************************
[bash][atlas]:krasznaa > which cppcheck
/cvmfs/atlas.cern.ch/repo/ATLASLocalRootBase/x86_64/cppcheck/2.6-x86_64-centos7-gcc11/cppcheck-2.6/bin/cppcheck
[bash][atlas]:krasznaa >

Note that I didn't remove the External/cppcheck package. It may still come in handy later on. I just stopped it from being used in any of the projects for now.

Pinging @christos and @sroe.