Removed invalid T template parameter from methods where it is not bound to any template

These functions had a `T` template parameter in them, but this `T` is completely decoupled
from the one defined at class-level (and therefore at `Enum` instance level, rather than
statically):

 * `Enum::from()`
 * `Enum::isValid()`
 * `Enum::assertValidValue()`
 * `Enum::assertValidValueReturningKey()` (internal detail)

In practice, this means that myclabs#135 (Added new named constructor to create enum from mixed)
was not a valid approach to infer types for enumerable values, when performed statically.

A new approach will be attempted, but the current one will likely need to be scrapped for
now.

In practice, `@psalm-assert`, `@psalm-assert-if-true` and `@psalm-return static<T>` had no
effect on these methods, due to a design-level issue that wasn't spotted (by myself either)
at review.

Author: Ocramius

src/Enum.php

@@ -95,7 +95,7 @@ public function __wakeup()
     /**
      * @param mixed $value
      * @return static
-     * @psalm-return static<T>
+     * @psalm-return static
      */
     public static function from($value): self
     {
@@ -212,7 +212,6 @@ public static function toArray()
      * @param $value
      * @psalm-param mixed $value
      * @psalm-pure
-     * @psalm-assert-if-true T $value
      * @return bool
      */
     public static function isValid($value)
@@ -224,7 +223,6 @@ public static function isValid($value)
      * Asserts valid enum value
      *
      * @psalm-pure
-     * @psalm-assert T $value
      * @param mixed $value
      */
     public static function assertValidValue($value): void
@@ -236,7 +234,6 @@ public static function assertValidValue($value): void
      * Asserts valid enum value
      *
      * @psalm-pure
-     * @psalm-assert T $value
      * @param mixed $value
      * @return string
      */