osl-languagespec.pdf no longer maintained/outdated references

[Sparsh-N]

Theres a few small references to tex based documentation which was recently removed to avoid duplication in the docs. In the main README.md, under the Documentation section there's mention of osl-languagespec.pdf which goes as follows:

The OSL language specification can be found at
[src/doc/osl-languagespec.pdf](src/doc/osl-languagespec.pdf) (in a source
distribution) or in the share/doc/OSL/osl-languagespec.pdf file of an
installed binary distribution.

It is compiled when building the docs locally in src/doc/Makefile, although latex for the languagespec is dropped in #1970 to reduce confusion regarding which file to update.

languagespec.pdf: *.tex Figures/*.pdf Figures/*png
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex
languagespec_index:
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex
	makeindex languagespec
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex languagespec.aux macros.aux
	pdflatex ${PDFLATEX_FLAGS} languagespec.tex languagespec.aux macros.aux languagespec.toc

A solution could be to modify the docs Makefile to go from .md -> .pdf, in place of the existing .tex -> .pdf. Alternatively, all mentions could be removed entirely if maintaining a pdf of the languagespec isn't needed.

[lgritz]

It would be great for somebody to clean this up. (No need to generate PDF, this is all done by readthedocs, just eliminate all the cruft that still refers to the old LaTeX path.)