Remove outdated comment on mc-cluster associations

[ruse-traveler]

Briefly, what does this PR introduce?

This PR removes an old comment left in the MCParticle-Cluster association code. The comment stated that the code doesn't work for HcalEndcapPClusters, However, this seems to be no longer the case.

What kind of change does this PR introduce?

• [X] Bug fix (issue #1387 )
• [ ] New feature (issue #__)
• [ ] Documentation update
• [ ] Other: __

Please check if this PR fulfills the following:

• [X] Tests for the changes have been added
• [ ] Documentation has been added / updated
• [X] Changes have been communicated to collaborators

Does this PR introduce breaking changes? What changes might users need to make to their code?

No.

Does this PR change default behavior?

No.

[github-actions[bot]]

Capybara summary for PR 1388

• rec_dis_10x100_minQ2=1000_craterlake
• rec_dis_10x100_minQ2=1_craterlake
• rec_dis_18x275_minQ2=1000_craterlake
• rec_dis_18x275_minQ2=1_craterlake
• rec_dis_5x41_minQ2=1_craterlake
• rec_e_1GeV_20GeV_craterlake
• rec_pi_1GeV_20GeV_craterlake ^{_{Last updated 2024-04-17T14:28-04:00 ba3abff462d96934079bdf10700bc73da9e1d834}}

[ruse-traveler]

Let's remove the FIXME comments. Issues need to be filed, if bugs are reappear in interplay of Digi and clustering.

Sounds good!