Remove redundant git-revision.txt file (#4881)

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

.gitignore

@@ -13,8 +13,7 @@ out
 /dist
 /lib
 
-# version file and tarball created by `npm pack` / `yarn pack`
-/git-revision.txt
+# tarball created by `npm pack` / `yarn pack`
 /matrix-js-sdk-*.tgz
 
 .vscode

package.json

@@ -9,8 +9,7 @@
         "prepare": "yarn build",
         "start": "echo THIS IS FOR LEGACY PURPOSES ONLY. && babel src -w -s -d lib --verbose --extensions \".ts,.js\"",
         "clean": "rimraf lib",
-        "build": "yarn build:dev",
-        "build:dev": "yarn clean && git rev-parse HEAD > git-revision.txt && yarn build:compile && yarn build:types",
+        "build": "yarn clean && yarn build:compile && yarn build:types",
         "build:types": "tsc -p tsconfig-build.json --emitDeclarationOnly",
         "build:compile": "babel -d lib --verbose --extensions \".ts,.js\" src",
         "gendoc": "typedoc",