Make custom fields text area larger

Fixes PLAT-4592

Test Plan:
When customizing a tool configuration, verify the custom fields
text area is large enought to easily read

Change-Id: If6a219bbe41aeed32fe996e4523d6f9b6c2f0f77
Reviewed-on: https://gerrit.instructure.com/197526
Reviewed-by: Marc Phillips <mphillips@instructure.com>
Tested-by: Jenkins
QA-Review: Marc Phillips <mphillips@instructure.com>
Product-Review: Jesse Poulos <jpoulos@instructure.com>

app/jsx/developer_keys/CustomizationForm.js

@@ -218,7 +218,6 @@ export default class CustomizationForm extends React.Component {
       <TextArea
         label={I18n.t('Custom Fields')}
         maxHeight="20rem"
-        width="50%"
         name="custom_fields"
         onChange={this.updateCustomFields}
         value={this.state.custom_fields}