gitignore: remove things that shouldn't be there

upstream libsecp now has a CMakeLists.txt file. Many years ago we added
some things to .gitignore which appear to be local developers committing
the names of their own stray files, and now this is causing the
revendoring script to lose track of vendored files.

Author: apoelstra

.gitignore

@@ -1,9 +1,2 @@
 target/
 Cargo.lock
-
-#IntelliJ project files
-.idea
-*.iml
-
-CMakeLists.txt
-cmake-build-debug