[TF2] Fix multiple inconsistency on Disposable Sentry Gun

[num-get]

Description

Gun Mettle update rebalanced the Mini Sentry, but unintentionally introduced multiple changes that cause the Disposable Sentry Gun to exhibit both the behavior of pre-2015 Mini Sentry and modern Mini Sentry.

• Disposable Sentry Gun starts at 100 HP as soon as it is built (same as Mini Sentry prior to 2015), but will self heal up to 50 HP during the construction process (same as current Mini Sentry).
• Play the denial sound during the construction process when hit by a wrench due to Disposable Sentry Gun used to not being able to be wrench boosted prior to 2015. This was only recently fixed last year. [TF2] Disposable Sentry only plays denial sound when not a bUsefulhit #1502 (comment)
• Inconsistent construction speed depending on whether the user is equipping the Gunslinger or the other wrenches https://youtu.be/WzJ9a4ZLBSU

To fix all the inconsistency, I simply restore the Disposable Sentry Gun to its older behavior prior to 2015.

• Remove self healing during the construction process.
• Remove the ability to wrench boost Disposable Sentry during the construction process, remove the spark effect while hitting it as well.
• Always build at 4x speed. Ignore the 2.5x sentry build speed attribute on the Gunslinger.
• 28 metals on dropped gibs.