refactor(core): drop unnecessary assignment of HOSTNAME

[pichlermarc]

Which problem is this PR solving?

We currently try to assign os.hostname() as the default, but we override it with the emtpy string from DEFAULT_ENVIRONMENT. This PR removes that assignment of HOSTNAME as it's not doing anything.

While we don't use HOSTNAME anywhere other than in tests, getEnv() may be used by consumers of the @opentelemetry/core library and they might depend on the default for that value being the empty string. Also since getEnv() indicates us getting environment variables, I think we should not fall back to anything that's not a static default (as is the case for all other variables).

Fixes #4405

Type of change

• refactor

How Has This Been Tested?

• Existing tests

[codecov]

Codecov Report

Merging #4421 (0431b98) into main (bf8714e) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 0431b98 differs from pull request most recent head 3141a92. Consider uploading reports for the commit 3141a92 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4421      +/-   ##
==========================================
+ Coverage   92.19%   92.21%   +0.02%     
==========================================
  Files         336      336              
  Lines        9512     9511       -1     
  Branches     2016     2016              
==========================================
+ Hits         8770     8771       +1     
+ Misses        742      740       -2     

Files	Coverage Δ
...pentelemetry-core/src/platform/node/environment.ts	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[legendecas]

LGTM % a nit