Update .gitignore after moving from YASM to NASM.

Change-Id: Ie0d8dc2942ffcd20fd8b62f75696986c97e5c327
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/68751
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

.gitignore

diff --git a/.gitignore b/.gitignore
index 67b2857..76b9b6a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,9 +16,8 @@
 /third_party/llvm
 /third_party/llvm-build
 /third_party/markupsafe
+/third_party/nasm
 /third_party/skia
-/third_party/yasm/binaries
-/third_party/yasm/source/patched-yasm
 /third_party/zlib
 /tools/clang
 /tools/code_coverage