LaTeX: stop using extractbb for image inclusion in Japanese documents

[jfbu]

Since TeXLive2015, the dvipdfmx binary does not need extra .xbb files
for images (which were produced using extractbb).

I propose to wait for Sphinx 3.0 because although a TeXLive2015-based TeX distribution is listed as requirement since Sphinx 2.0, it is not needed to really force users to upgrade only to help us trim a few lines on our side which are neither an added feature nor a bugfix but simply some steps which were mandatory in the past and are now not needed.

Or at least wait until we shift from platex to uplatex for Japanese projects (refs: #4187 (comment))

edit: I forgot to add this link for source of information about extractbb unneeded since TL2015.

[codecov]

Codecov Report

Merging #6189 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6189   +/-   ##
=======================================
  Coverage   83.47%   83.47%           
=======================================
  Files         275      275           
  Lines       39461    39461           
  Branches     5886     5886           
=======================================
  Hits        32941    32941           
  Misses       5167     5167           
  Partials     1353     1353

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[wizardwhite2009]

This is all Greek to me, but I personally have all of Adobes products.

[tk0miya]

+1, but current master branch is going forward v2.1.
At present, no appropriate branch for v3.0 there.

[tk0miya]

Now master branch is ready for 3.0!

[jfbu]

Thanks for reviewing! merging now that master is moving towards 3.0!