Use top-level directory gradle and delete from third_party (flutter#8397)

Gradle files should generally be at the top level and the jar file
should be committed:
https://docs.gradle.org/current/userguide/gradle_wrapper.html#sec:adding_wrapper

We also want to check that the gradle jar file matches the checksums
listed:
https://docs.gradle.org/current/userguide/gradle_wrapper.html#wrapper_checksum_verification
(Added flutter#8396 to do
this automatically in the future)

I verified that our dev build works with using the new top-level files
instead of calling `./third_party/gradlew`

Author: helin24

.gitignore

@@ -47,4 +47,3 @@ lib/kotlin*
 .gradle
 /build/
 gradle-app.setting
-gradle/wrapper/gradle-wrapper.jar

build.gradle.kts

@@ -282,7 +282,7 @@ tasks {
 }
 
 // A task to print the classpath used for compiling an IntelliJ plugin
-// Run with `./third_party/gradlew printCompileClasspath --no-configuration-cache `
+// Run with `./gradlew printCompileClasspath --no-configuration-cache `
 tasks.register("printCompileClasspath") {
   doLast {
     println("--- Begin Compile Classpath ---")
@@ -314,8 +314,6 @@ val writeLicenseKey = tasks.register("writeLicenseKey") {
     if (readFile.isFile) {
       val licenseKey = readFile.readText(Charsets.UTF_8)
       licenseFile.writeText("jxbrowser.license.key=$licenseKey")
-    } else {
-      println("$readFile is not a file")
     }
   }
 }

gradle/wrapper/gradle-wrapper.jar

@@ -5,6 +5,6 @@ setup
 
 echo "kokoro build start"
 
-./third_party/gradlew buildPlugin
+./gradlew buildPlugin
 
 echo "kokoro build finished"

third_party/gradle-LICENSE.txt

@@ -5,7 +5,7 @@ setup
 
 echo "kokoro build start"
 
-./third_party/gradlew buildPlugin -Pdev-version=88.0
+./gradlew buildPlugin -Pdev-version=88.0
 
 echo "kokoro build finished"