Remove travis config

[palfrey]

On the basis that https://app.travis-ci.com/github/quickwit-inc/tantivy isn't activated and there's a Github Actions setup, I'm assuming the travis config isn't needed anymore?

[codecov-commenter]

Codecov Report

Merging #1180 (76fcd95) into main (3a78402) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1180      +/-   ##
==========================================
- Coverage   93.98%   93.98%   -0.01%     
==========================================
  Files         204      204              
  Lines       34539    34541       +2     
==========================================
  Hits        32462    32462              
- Misses       2077     2079       +2     

Impacted Files	Coverage Δ
src/directory/file_watcher.rs	96.69% <0.00%> (-3.31%)	⬇️
fastfield_codecs/src/multilinearinterpol.rs	98.03% <0.00%> (+0.01%)	⬆️
src/store/index/mod.rs	98.39% <0.00%> (+0.53%)	⬆️
src/directory/directory.rs	95.16% <0.00%> (+1.61%)	⬆️

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[fulmicoton]

Very good question! @PSeitz are we done with Travis?

[PSeitz]

Yep, we can delete it

[fulmicoton]

Thank you @palfrey