Remove top-level pf folder

[guineveresaenger]

We have migrated the PF functionality to live inside of pkg/pf instead of its own module.

This pull request removes the deprecated files.

Reference:
#2480
pulumi/ci-mgmt#1103

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.97%. Comparing base (c7d87ed) to head (02367c5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2653   +/-   ##
=======================================
  Coverage   69.97%   69.97%           
=======================================
  Files         367      367           
  Lines       44504    44504           
=======================================
+ Hits        31141    31142    +1     
- Misses      11734    11735    +1     
+ Partials     1629     1627    -2     

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[blampe]

There are still a couple things to do before it's "safe" to remove this:

1. Cut one final pf/... (and x/... and testing/..) tag so users will pick up the "Deprecated" tag on the module.
2. Wait a sufficiently long amount of time for users to notice pf is deprecated and point at the new module.

Step (2) means technically we might want to keep this around forever to avoid breaking third-parties years from now.

[guineveresaenger]

Summary of internal discussion:

• Cut a final deprecated release for pf
• Then remove this code(?)

[iwahbe]

We need to wait at least one more release, then verify that this won't break people before merging.