Revert "Use proguard version 6.0.3 for monochrome_public_apk."

This reverts commit df22b814.

Reason for revert: breaks generate_build_files (http://crbug.com/869231)

Original change's description:
> Use proguard version 6.0.3 for monochrome_public_apk.
> 
> MonochromePublic.apk release builds are very close to the main dex limit
> and updating proguard reduces the number of methods by about 3000.
> 
> 6.0.3 cannot be used for all APKs yet due to a bug which causes
> cronet_test_instrumentation_apk to fail.
> 
> The version mismatch should be temporary as we plan to switch to R8 in the
> future (https://crbug.com/868770).
> 
> Bug: 857572
> Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
> Change-Id: I3676a42908c5046dc3f3831e282150702038140d
> Reviewed-on: https://chromium-review.googlesource.com/1154366
> Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
> Commit-Queue: Eric Stevenson <estevenson@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#579250}

TBR=yfriedman@chromium.org,estevenson@chromium.org

Change-Id: I1d7e50cf720e9a128a62f06bb34a4c9f6d25c60d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 857572
Bug: 869231
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Reviewed-on: https://chromium-review.googlesource.com/1155502Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579262}

build/config/android/rules.gni

@@ -2919,7 +2919,6 @@ if (enable_java_templates) {
                                "product_version_resources_dep",
                                "proguard_configs",
                                "proguard_enabled",
-                               "proguard_jar_path",
                                "requires_sdk_api_level_23",
                                "secondary_abi_loadable_modules",
                                "secondary_abi_shared_libraries",

chrome/android/chrome_public_apk_tmpl.gni

@@ -197,8 +197,6 @@ template("monochrome_public_apk_tmpl") {
     }
 
     if (!is_java_debug) {
-      # TODO(crbug.com/857572): Make all upstream APKs use the same proguard version.
-      proguard_jar_path = "//third_party/proguard/lib/proguard.6.0.3.jar"
       if (!defined(proguard_configs)) {
         proguard_configs = []
       }

third_party/proguard/README.chromium

@@ -10,6 +10,3 @@ Description:
 This directory includes proguard.jar to allow Chromium to shrink, optimize java
 classes for Android, as well as retrace.jar, to allow for deobfuscation of stack
 traces.
-
-Note: 6.0.3 is used for monochrome_public_apk to avoid hitting the dex limit
-(crbug.com/857572).