Refactor plotting routines + remove unused functionality

This PR tackles two main aspects regarding general refactoring and cleanup, with the hope that this will make it easier to make improvements/contributions, particularly with regards to the plotting and summary pages.

1. Remove old and unused functionality:

The goal here is to remove old functionality that hasn't been used in years:

• Remove IIR, SPIIR related functionality: At this point, all of the SPIIR codebase has migrated to another repository and all IIR-related code that I can identify are SPIIR-specific and would need some significant work for it to be useful again.
• Remove cgi-based low latency summary: All of this has been replaced by ligo-scald for the online analysis.
• Trim all DAX/Pegasus stuff in pipeline.py which is unused by the analysis.

2. Restructure/refactor plotting utilities:

This is in part a migration to move plot modules (plotX.py) into a plots submodule as plots/X.py, which will give some compartmentalization similarly to gstlal.stats. This will have the consequence of having imports being changed, but behavior in any external modules/scripts depending on it can do the following to minimize impact:

from gstlal import plotutil -> from gstlal.plots import util as plotutil

The second part is to refactor some of the plotting routines which only existed in standalone scripts and have them be properly importable. These include:

• gstlal_inspiral_plot_banks -> gstlal.plots.bank
• gstlal_inspiral_plot_sensitivity -> gstlal.plots.sensitivity
• gstlal_inspiral_plot_summary -> gstlal.plots.summary