Remove redundant and very low quality missions

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Since the target practice missions (see here: 
http://www.hedgewars.org/node/5958) have been merged recently, there are a few 
redundant missions in the game now.

Namely, the following:
- Basic Training: Cluster Bomb
→ This mission has been reimplemented as Target Practice: Cluster Bomb, it is 
now completely needless now.

- Basic Training: Shotgun
→ This mission is of poor quality. The targets spawn at random positions and 
sometimes it is literally impossible to even reach the line of sights of all 5 
targets in time. Also, there is an alternative with Target Practice: Shotgun, 
although this mission is not equivalent

Please remove both of them. Feel free to discuss if you want to remove other 
old/broken missions.

Original issue reported on code.google.com by almikes@aol.com on 15 Dec 2014 at 2:24

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Original comment by sheepyluva on 16 Jul 2015 at 3:42

• Changed state: Accepted

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addressed in r93b3870a538d - thanks!

Original comment by sheepyluva on 16 Jul 2015 at 8:01