[cleanup] Remove entries from .gitignore

I removed the following entries:
.d8_history - afaict, this does not exist anymore. If it exists, it
              would be in the out/ directory and thereby ignored anyways.
/hydrogen.cfg - hydrogen is no more.
/obj - this is now also in the out/ directory.
d8_g - I think this does not exist anymore, or would be in the out/
       directory.
shell - If it still exists, it would be in the out/ directory.
shell_g - same here.

NOTRY=true
R=jkummerow@chromium.org

Bug: v8:9183
Change-Id: Iadb8d2533bfdda1190ec3a31a9ca9c199e087fd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1660468
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62166}

.gitignore

@@ -26,7 +26,6 @@
 .ccls-cache
 .cpplint-cache
 .cproject
-.d8_history
 .gclient_entries
 .gdb_history
 .landmines
@@ -40,8 +39,6 @@
 /build
 /buildtools
 /check-header-includes
-/hydrogen.cfg
-/obj
 /out
 /out.gn
 /perf.data
@@ -88,13 +85,9 @@ GTAGS
 TAGS
 bsuite
 compile_commands.json
-!/test/mjsunit/d8
-d8_g
 gccauses
 gcsuspects
 gtags.files
-shell
-shell_g
 tags
 turbo*.cfg
 turbo*.dot