chore: Remove old Beamer .tex lecture files

All lectures have been migrated to Marp markdown format with CDL theme.
The .tex files are no longer needed and have been removed:
- slides/week1/lecture{1,2,3}.tex
- slides/week3/lecture{7,8}.tex
- slides/week4/lecture{9,10,11}.tex
- slides/week5/lecture{12,13,14}.tex
- slides/week6/lecture{15,16,17}.tex
- slides/week7/lecture{18,19,20}.tex
- slides/week9/lecture{21,22,23}.tex
- slides/week10/lecture24.tex
- Various meta lecture.tex files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jeremymanning