Remove Access-Control-Allow-Origin

[jvoisin]

It doesn't server any purpose, and allows website to
probe if AdGuard Home is currently running on the LAN of the
user by bruteforcing common IP addresses (192.168.1.0/24 and 192.168.0.0/24)
until one of them returns AGH's html.

[codecov-io]

Codecov Report

Merging #2484 (b1c7ebd) into master (e829e7a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2484   +/-   ##
=======================================
  Coverage   38.66%   38.66%           
=======================================
  Files          84       84           
  Lines        9471     9470    -1     
=======================================
  Hits         3662     3662           
+ Misses       5351     5350    -1     
  Partials      458      458           

Impacted Files	Coverage Δ
internal/home/control.go	0.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e829e7a...b1c7ebd. Read the comment docs.

[ameshkov]

@ainar-g please check the commits history. I don't remember why we added, but I believe there was a reason for this.

[ainar-g]

Thanks for the contribution! I've analyzed the history of that line and discovered that we do need to set the header in some cases (the main one being the state after the user enables HTTPS redirect) but probably not to *. (And the code also required some refactoring.) See bc01f4f. I will close this PR, but feel free to reopen if you think that something else could be improved there.