perf: no longer generate unnecessary nginx conf for better performance.

[membphis]

What this PR does / why we need it:

as title.

Single-core QPS increased from 20118.64 to 22448.23, an increase of about 12%.
And the QPS of this version is 89% of fake apisix.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible?
  • disable two plugins by default(proxy-cache, proxy-mirror), if the user wants to enable them, need to modify the conf/config.yaml by manual.
  • has been reverted: revert: enable proxy-cache and proxy-mirror plugins by default. #1992

[membphis]

Talked with @moonming right now, will enable proxy-cache, proxy-mirror by default in APISIX.
For most users, it is easier to use API​​SIX.

Add a new document named What are the best practices for benchmarking APISIX?, tell the user how do benchmark for APISIX.