Clean up sitemap.xml and robots.txt implementation

[lpsinger]

I forked https://github.com/nasa-gcn/remix-seo from https://github.com/balavishnuvj/remix-seo because the latter may be abandoned.

This PR eliminates the overriden Remix server entry point (entry.server.tsx) and instead has ordinary resource route modules for the robots.txt and sitemap.xml files.

See also balavishnuvj/remix-seo#14

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.11% 🎉

Comparison is base (3b53bd3) 5.64% compared to head (2ecee53) 5.75%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1355      +/-   ##
========================================
+ Coverage   5.64%   5.75%   +0.11%     
========================================
  Files        112     112              
  Lines       2606    2554      -52     
  Branches     278     273       -5     
========================================
  Hits         147     147              
+ Misses      2457    2405      -52     
  Partials       2       2              

Files Changed	Coverage Δ
app/routes/$.ts	0.00% <ø> (ø)
app/routes/_auth.post_logout.tsx	0.00% <ø> (ø)
app/routes/_seo.robots[.]txt.ts	0.00% <0.00%> (ø)
app/routes/_seo.sitemap[.]xml.ts	0.00% <0.00%> (ø)
app/routes/quickstart.alerts.tsx	0.00% <ø> (ø)
app/routes/quickstart.code.tsx	0.00% <ø> (ø)
app/routes/quickstart.credentials.tsx	0.00% <ø> (ø)
app/routes/unsubscribe.$jwt/route.tsx	0.00% <ø> (ø)
app/routes/user._index.tsx	0.00% <ø> (ø)
app/routes/user.credentials._index.tsx	0.00% <ø> (ø)
... and 8 more

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