bugfix: dead end connection becomes disabled when removed.

[kianzarrin]

fixes #1678
code is simple and self explanatory.

changes:

• added extra condition to handle removal of special case of dead end.
• improved lane assertion to check for disabled dead end.

TMPE.zip

I Tested the code like this:

• make and remove dead end connection
• then make and remove connection to another track
• then make connection to the other track and remove it by making a dead end connection.
• then make a dead end connection and remove it by making connection to the other track
  expected result:
• tracks are connected and disconnected as expected
• no assertion/exception/error in the log.

[kvakvs]

Tested, no errors in log, worked as expected

[chameleon-tbn]

all seem to work as expected. 👍

[kianzarrin]

@krzychu124 can you please push a hotfix for this?