Cleanup GetEnvironmentVariable and remove unused variable under NO_GETENV

[ThomsonTan]

Changes

When NO_GETENV is used, it is unnecessary to define a pointer to save the result as it is always nullptr.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #976 (1b0cb55) into main (fc8f854) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files         161      161           
  Lines        6817     6817           
=======================================
  Hits         6502     6502           
  Misses        315      315           

Impacted Files	Coverage Δ
...k/include/opentelemetry/sdk/common/env_variables.h	100.00% <100.00%> (ø)