chore: remove the useless configuration in the ngx_tpl.lua file

[Firstsawyou]

What this PR does / why we need it:

close #3662

Since APISIX and APISIX Dashboard are currently deployed independently of each other, the old /apisix/dashboard API interface is still retained in the ngx_tpl.lua file. We should delete it.
The reasons are as follows:

The /apisix/dashboard API interface is useless;
When users use the rpm package to install APISIX and APISIX Dashboard, there is a directory structure named dashboard in the directory structure of APISIX, which is easy for users to misunderstand that they need to use the /apisix/dashboard API interface to perform access.

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?
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