MNT: cleanup lit

Remove file entries.

book/lit.bib

@@ -19,7 +19,6 @@ @Online{goertler_2019_VisualExplorationGaussiana
   urldate      = {2022-11-22},
   abstract     = {How to turn a collection of small building blocks into a versatile tool for solving regression problems.},
   doi          = {10.23915/distill.00017},
-  file         = {/home/elcorto/.zotero/storage/SALHLB8Q/visual-exploration-gaussian-processes.html},
   issn         = {2476-0757},
   journaltitle = {Distill},
   keywords     = {gp_play},
@@ -38,7 +37,6 @@ @Online{kanagawa_2018_GaussianProcessesKernel
   archiveprefix = {arXiv},
   eprint        = {1807.02582},
   eprinttype    = {arxiv},
-  file          = {files/papers/Kanagawa et al. - 2018 - Gaussian Processes and Kernel Methods A Review on.pdf;/home/elcorto/.zotero/storage/ZZXBTU3Q/1807.html},
   keywords      = {gp_play},
   primaryclass  = {cs, stat},
   shorttitle    = {Gaussian {{Processes}} and {{Kernel Methods}}},
@@ -50,7 +48,6 @@ @Book{murphy_2023_ProbabilisticMachineLearningAdvanced
   year      = {2023},
   publisher = {MIT Press},
   url       = {https://probml.github.io/pml-book/book2.html},
-  file      = {:files/books/pml_book_2.pdf:PDF},
   keywords  = {gp_play, mlbook},
 }
 
@@ -61,7 +58,6 @@ @Book{rasmussen_2006_GaussianProcessesMachine
   publisher = {MIT Press},
   isbn      = {ISBN-10 0-262-18253-X},
   url       = {http://www.gaussianprocess.org/gpml},
-  file      = {:files/books/Rasmussen_Williams_2006.pdf:PDF},
   keywords  = {gp_play},
 }