Remove unused SPHINXPROJ from new Makefile/make.bat.

[anntzer]

Subject: Remove unused SPHINXPROJ from new Makefile/make.bat.

Feature or Bugfix

Bugfix

Purpose

The SPHINXPROJ makefile variable has become unused since #4354. Remove it.

Detail

See above.

Relates

N/A

[codecov]

Codecov Report

Merging #4954 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4954      +/-   ##
==========================================
- Coverage   82.22%   82.21%   -0.01%     
==========================================
  Files         294      289       -5     
  Lines       38635    38313     -322     
  Branches     5953     5899      -54     
==========================================
- Hits        31766    31498     -268     
+ Misses       5547     5500      -47     
+ Partials     1322     1315       -7

Impacted Files	Coverage Δ
sphinx/make_mode.py	1.12% <ø> (+0.01%)	⬆️
sphinx/builders/html.py	82.37% <0%> (-0.12%)	⬇️
sphinx/quickstart.py
sphinx/__init__.py
sphinx/search/__init__.py
sphinx/errors.py
sphinx/apidoc.py

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 169297d...f101bb6. Read the comment docs.

[tk0miya]

Thanks!