MNT Remove unused dev-dependency

[GuySartorelli]

Fixes https://github.com/silverstripe/silverstripe-mimevalidator/actions/runs/11061570379/job/30734529021

  Problem 1
    - Root composer.json requires silverstripe/framework ^6 -> satisfiable by silverstripe/framework[6.x-dev].
    - silverstripe/framework 6.x-dev requires nikic/php-parser ^5.1.0 -> found nikic/php-parser[v5.1.0, v5.2.0] but it conflicts with your root composer.json require (^4.15.0).
  Problem 2
    - silverstripe/framework 6.x-dev requires nikic/php-parser ^5.1.0 -> found nikic/php-parser[v5.1.0, v5.2.0] but it conflicts with your root composer.json require (^4.15.0).
    - silverstripe/installer 6.x-dev requires silverstripe/login-forms 6.x-dev -> satisfiable by silverstripe/login-forms[6.x-dev].
    - silverstripe/login-forms 6.x-dev requires silverstripe/framework ^6 -> satisfiable by silverstripe/framework[6.x-dev].
    - Root composer.json requires silverstripe/installer 6.x-dev -> satisfiable by silverstripe/installer[6.x-dev].

nikic/php-parser dev dependency doesn't seem to even be used, so I've just removed it outright.

Issue

• Broken builds and merge-ups .github#313