[cli] Remove the never used 'tid' command

[archibate]

Related issue = #

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1. Remove the tid command:
   We already have ti debug and the debug argument in main seems useless...
   In fact, it's already not functional at all:

root@archlinux ~/taichi (git)-[remove-tid-useless-bin-hook] # tid
[Taichi] mode=development
[Taichi] preparing sandbox at /tmp/taichi-6z2hiodm
[Taichi] <dev mode>, llvm 10.0.0, commit a93efc54, python 3.8.3
Traceback (most recent call last):
  File "/root/taichi/bin/tid", line 4, in <module>
    exit(taichi.main(debug=True))
TypeError: main() got an unexpected keyword argument 'debug'

2. Use #!/usr/bin/env python3 instead of #!${PYTHON_EXECUTABLE}:
   Well, what if one use anaconda python3?
   The PYTHON_EXECUTABLE is determinated by the buildbots, I don't think it's always the same as end-users...
   In contrast, /usr/bin/env will automatically search for python3 in current PATH, which will make ti command work in anaconda python3.
   @k-ye Just to confirm, is /usr/bin/env always available on Mac?

3. Make Linux users without CUDA able to use ti dist.
   I thought our Linux buildbot have constant CUDA support? So no need for this confirm?

[rexwangcc]

LGTM!

1. It seems tid works in my environment:

(py-taichi-dev) ➜  ~ tid
[Taichi] mode=development
[Taichi] preparing sandbox at /var/folders/jr/_8tzt9ys77z10wt2kx09vyg40000gn/T/taichi-dsvhetv7
[Taichi] <dev mode>, llvm 8.0.1, commit f920fe99, python 3.7.3

*******************************************
**      Taichi Programming Language      **
*******************************************

***************** Debug Mode *****************

usage:
    baseline            |-> Archive current benchmark result as baseline

but if it's not being used by any stuff, 👍 to remove it for clarity.

2. I believe most Unix systems provided env in /usr/bin, including macOS, but I'm not sure if itimproves the protability as I haven't checked how PYTHON_EXECUTABLE_PATH is generated now.

[archibate]

Just to confirm: @yuanming-hu Does this have fatal drawback?
I just want to run ti dist on my non-CUDA laptop.

[archibate]

Merging this given that v0.6.19 is already released, ff2 revert me if you don't like this.