Remove _always_use_native=False

[VivekPanyam]

Summary:

_always_use_native=False was always intended to be a workaround (hence the leading _) and has had a deprecation warning since #508.

Removing this functionality allows us to delete a lot of code and have a single inference code path across C, C++, Java, Python, and Go.

Test Plan:

CI

[VivekPanyam]

cc @csashiuber @qiyanz @vkuzmin-uber

[codecov[bot]]

Codecov Report

Merging #526 (8919b7b) into master (a47d1c8) will decrease coverage by 0.64%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
- Coverage   87.21%   86.56%   -0.65%     
==========================================
  Files         111      105       -6     
  Lines        7289     6930     -359     
==========================================
- Hits         6357     5999     -358     
+ Misses        932      931       -1     

Impacted Files	Coverage Δ
source/python/neuropod/loader.py	65.15% <ø> (-29.92%)	:arrow_down:
source/python/neuropod/utils/eval_utils.py	78.37% <100.00%> (-3.02%)	:arrow_down:
source/python/neuropod/utils/config_utils.py	88.46% <0.00%> (-6.42%)	:arrow_down:
source/python/neuropod/utils/env_utils.py

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