MAINT: Ignore .pxi files

All `.pxi` files have the warning to not edit directly, so why not
have Git ignore any changes made there?    In addition, this PR
deletes all of the `.pxi` files from the codebase because they are
automatically generated by the `.pxi.in` files

Author: gfyoung <[email protected]>

Closes pandas-dev#14723 from gfyoung/gitignore-pxi-ignore and squashes the following commits:

fe0c410 [gfyoung] MAINT: Ignore .pxi files

Author: gfyoung

.gitignore

@@ -27,6 +27,7 @@
 *.class
 *.dll
 *.exe
+*.pxi
 *.o
 *.py[ocd]
 *.so