live-ai: Update to use FLV-enabled LPMS

[j0sh]

What does this pull request do? Explain your changes. (required)

Updates LPMS to use the FLV muxer introduced in livepeer/lpms#428 which should resolve issues with getting RTMP output.

Specific updates (required)

• Updates LPMS to b33cac634b43d2ecd160224417daf8e920b0f500
• Uses LPMS for ffmpeg output
• Remove ffmpeg from go-livepeer Dockerfile

How did you test each of these updates (required)

Manually tested a production LPMS build with and without the FLV muxer.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 34.48947%. Comparing base (baf0383) to head (9981665).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
server/ai_live_video.go	0.00000%	12 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3288         +/-   ##
===================================================
+ Coverage   34.48597%   34.48947%   +0.00350%     
===================================================
  Files            137         137                 
  Lines          36496       36501          +5     
===================================================
+ Hits           12586       12589          +3     
- Misses         23194       23196          +2     
  Partials         716         716                 

Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <0.00000%> (ø)

... and 1 file with indirect coverage changes

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Files with missing lines	Coverage Δ
server/ai_live_video.go	0.00000% <0.00000%> (ø)

... and 1 file with indirect coverage changes