Remove post install message

Author: djmb

solid_cache.gemspec

@@ -12,11 +12,6 @@ Gem::Specification.new do |spec|
   spec.description = "A database backed ActiveSupport::Cache::Store"
   spec.license     = "MIT"
 
-  spec.post_install_message = <<~MESSAGE
-    Upgrading from Solid Cache v0.3 or earlier? There are new database migrations in v0.4.
-    See https://github.com/rails/solid_cache/blob/main/upgrading_to_version_0.4.x.md for upgrade instructions.
-  MESSAGE
-
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "http://github.com/rails/solid_cache"