build: drop the unused solana dependency, whose source repo is gone

github.com/espresso-cash/espresso-cash-public now answers "Repository
not found", so every pub get on a runner without a warm cache fails
cloning it, and all build 17 jobs died generating the workspace. Hash
Bags configures no Solana module and no Dart code imports solana or
borsh_annotation; the base pubspec and two overrides were all that
named them. Upstream removed the same pins in #3814 (15d591f1b); its
cherry-pick conflicts with our pruned module tree, so this takes only
the solana part.

pubspec_base.yaml

@@ -116,7 +116,6 @@ dependencies:
     git:
       url: https://github.com/cake-tech/nostr_tools.git
       ref: 089d5a2dd751429a040ba10fb24fcbae564053e5
-  solana: ^0.31.0+1
   ledger_flutter_plus:
     git:
       url: https://github.com/vespr-wallet/ledger-flutter-plus

pubspec_overrides.yaml

@@ -112,11 +112,6 @@ dependency_overrides:
       url: https://github.com/dart-lang/tools
       ref: b1312b18df67b0be3101ca15c03dfd8d7c9383bb
       path: ./pkgs/boolean_selector
-  borsh_annotation:
-    git:
-      url: https://github.com/espresso-cash/espresso-cash-public
-      ref: ab60e9d4c7d7271037597154ad2a835812054ed6
-      path: ./packages/borsh_annotation
   breez_sdk_spark_flutter:
     git:
       url: https://github.com/breez/breez-sdk-spark-flutter
@@ -1339,11 +1334,6 @@ dependency_overrides:
     git:
       url: https://github.com/sneurlax/socks_socket
       ref: e6232c53c1595469931ababa878759a067c02e94
-  solana:
-    git:
-      url: https://github.com/espresso-cash/espresso-cash-public
-      ref: ab60e9d4c7d7271037597154ad2a835812054ed6
-      path: ./packages/solana
   source_gen:
     git:
       url: https://github.com/dart-lang/source_gen