Update getType() to use $this->singular_name() for extensibility

[jsirish]

Summary

Update getType() to allow site-specific customization via extensions.

Current Implementation

public function getType()
{
    return _t(__CLASS__ . '.BlockType', 'Testimonials');
}

Proposed Change

Remove the getType() override entirely. The base BaseElement::getType() already uses $this->i18n_singular_name() which respects $singular_name:

// BaseElement::getType() - already does the right thing
public function getType()
{
    $default = $this->i18n_singular_name() ?: 'Block';
    return _t(static::class . '.BlockType', $default);
}

By removing the override, the element will inherit this behavior and allow sites to customize the display name via extensions by simply setting $singular_name.

Rationale

This pattern allows sites using these elements to customize the display name in the CMS element picker by simply setting $singular_name via an extension, rather than having to override the entire getType() method (which is not possible via extensions since PHP calls the class method directly, bypassing __call()).

Related: dynamic/silverstripe-essentials-tools#68