Move most raster layers to MaplibreGL & remove dead code

[ZeLonewolf]

Initial incremental step towards OpenHistoricalMap/ohm-website#651

Description

This PR moves most of the raster layers to MapLibre GL and removes associated dead code that I discovered during the process. I'm leaving the standard tile layer for now because it's hooked into a few other places that seemed more complicated to handle.

How has this been tested?

Tested locally, appears to look identically between localhost and openhistoricalmap.org.

[1ec5]

This will always resolve to "disable". The ternary is apparently unnecessary because we don’t feature the Tracestrack Topo layer, but will this be a source of conflicts with upstream?

[1ec5]

Nit: This part of the diff is somewhat inconsistent in its use of single versus double quotation marks.

[ZeLonewolf]

This is like this because the file I pulled it in from was using single quotes. Should I just change them to be consistent through?

[1ec5]

Yeah, might as well to be internally consistent.

[danrademacher]

@erictheise is going to look at this as soon as we do an upstream catchup, expected today. Then we'll look at this relative to that newer code from upstream

[1ec5]

#316 eliminated some of the custom maplibre-gl-leaflet integration code in favor of what openstreetmap-website did upstream in openstreetmap#6137. It’s cleaner and more reliable, though there’s some additional code to clean up around how the time slider hooks in. This refactoring conflicts with your PR, but it looks like most of the changes are effectively merged already. The one thing that hasn’t landed is the commented-out option to specify a Tracestrack API key.