Remove the allowUninstall policy option and clean up the only usage

src/Composer/DependencyResolver/DefaultPolicy.php

@@ -21,11 +21,6 @@ use Composer\Package\LinkConstraint\VersionConstraint;
  */
 class DefaultPolicy implements PolicyInterface
 {
-    public function allowUninstall()
-    {
-        return true;
-    }
-
     public function versionCompare(PackageInterface $a, PackageInterface $b, $operator)
     {
         $constraint = new VersionConstraint($operator, $b->getVersion());

src/Composer/DependencyResolver/PolicyInterface.php

@@ -20,7 +20,6 @@ use Composer\Package\PackageInterface;
  */
 interface PolicyInterface
 {
-    function allowUninstall();
     function versionCompare(PackageInterface $a, PackageInterface $b, $operator);
     function findUpdatePackages(Solver $solver, Pool $pool, array $installedMap, PackageInterface $package);
     function installable(Solver $solver, Pool $pool, array $installedMap, PackageInterface $package);

src/Composer/DependencyResolver/Solver.php

@@ -986,14 +986,9 @@ class Solver
             $updates = $this->policy->findUpdatePackages($this, $this->pool, $this->installedMap, $package);
             $rule = $this->createUpdateRule($package, $updates, self::RULE_INTERNAL_ALLOW_UPDATE, (string) $package);
 
-            if ($this->policy->allowUninstall()) {
-                $rule->setWeak(true);
-                $this->addRule(RuleSet::TYPE_FEATURE, $rule);
-                $this->packageToFeatureRule[$package->getId()] = $rule;
-            } else {
-                $this->addRule(RuleSet::TYPE_UPDATE, $rule);
-                $this->packageToUpdateRule[$package->getId()] = $rule;
-            }
+            $rule->setWeak(true);
+            $this->addRule(RuleSet::TYPE_FEATURE, $rule);
+            $this->packageToFeatureRule[$package->getId()] = $rule;
         }
 
         foreach ($this->jobs as $job) {