Remove duplicate debug logs when using task.exec* wrappers

[KonstantinTyukalov]

Issue #1069

We already subscribe to debug events in the tool function

So no need to do this twice as it leads to debug logs duplication.

Before:

##vso[task.debug]exec tool: C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe
##vso[task.debug]exec tool: C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe
##vso[task.debug]arguments:
##vso[task.debug]arguments:
##vso[task.debug]   -c
##vso[task.debug]   -c
##vso[task.debug]   echo hello
##vso[task.debug]   echo hello
[command]C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe -c "echo hello"
hello
##vso[task.debug]Process exited with code 0 and signal null for tool 'C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe'
##vso[task.debug]Process exited with code 0 and signal null for tool 'C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe'

after:

##vso[task.debug]exec tool: C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe
##vso[task.debug]arguments:
##vso[task.debug]   -c
##vso[task.debug]   echo hello
[command]C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe -c "echo hello"
hello
##vso[task.debug]Process exited with code 0 and signal null for tool 'C:\windows\System32\WindowsPowerShell\v1.0\powershell.exe'