Remove references to centos8 rpm packaging

[egieseke]

Summary

Remove old references to centos8 rpm packaging. There is now a single package for both centos 7 & 8.

Test Plan

make clean, make install, make test. Validate PR testing completes successfully.

[codecov-commenter]

Codecov Report

Merging #2605 (351a770) into master (02eacd1) will increase coverage by 0.21%.
The diff coverage is 54.11%.

@@            Coverage Diff             @@
##           master    #2605      +/-   ##
==========================================
+ Coverage   46.72%   46.94%   +0.21%     
==========================================
  Files         344      348       +4     
  Lines       55256    55715     +459     
==========================================
+ Hits        25817    26153     +336     
- Misses      26508    26613     +105     
- Partials     2931     2949      +18     

Impacted Files	Coverage Δ
agreement/abstractions.go	50.00% <ø> (ø)
agreement/proposalManager.go	96.07% <ø> (ø)
agreement/voteTracker.go	90.54% <ø> (ø)
catchup/catchpointService.go	0.00% <0.00%> (ø)
cmd/goal/account.go	14.73% <0.00%> (-0.09%)	⬇️
cmd/goal/application.go	12.86% <0.00%> (-0.52%)	⬇️
cmd/goal/asset.go	16.29% <0.00%> (-0.72%)	⬇️
cmd/goal/clerk.go	9.14% <ø> (ø)
cmd/goal/interact.go	3.36% <0.00%> (-0.03%)	⬇️
cmd/tealdbg/cdtSession.go	67.87% <0.00%> (ø)
... and 91 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9bcee44...351a770. Read the comment docs.

[algobarb]

Looks like it passed all the tests and removed all the lines we had an issue with when running the beta rpm deploy. Looks good to me.