Start cleanup of REMARK - unlink submodules

[MridulS]

REMARK repo is supposed to be a index of REMARK, in this PR we remove all the submodules and just link them from a markdown file.

• Removed cAndCwithStickyE submodule, a link from cAndCwithStickyE.md points towards the repo https://github.com/llorracc/cAndCwithStickyE
• Removed ctDiscrete submodule
• Removed cstwMPC submodule
• Removed BufferStockTheory submodule
• Removed LiqConstr submodule
• Removed Pandemic submodule