Simplify client transports

[acolytec3]

This PR entirely removes the transports CLI parameter and has a number of related changes:

• Networking (i.e. RLPx server and discv4/dns discovery) is now not activated when --sync=none is passed in the command line
• Converts the servers property of the Config type into a single server and adjusts related code
• Removes the transports property from Config cleans up boatloads of tests
  where the transports property was set

[codecov]

Codecov Report

Merging #3069 (2eab383) into master (f99a295) will increase coverage by 0.00%.
The diff coverage is 76.92%.

Additional details and impacted files

Flag	Coverage Δ
block	88.78% <ø> (ø)
blockchain	92.61% <ø> (ø)
client	87.78% <76.92%> (-0.04%)	⬇️
common	98.19% <ø> (ø)
ethash	∅ <ø> (∅)
evm	71.87% <ø> (ø)
rlp	∅ <ø> (∅)
statemanager	90.17% <ø> (ø)
trie	90.55% <ø> (+0.23%)	⬆️
tx	96.36% <ø> (ø)
util	86.97% <ø> (ø)
vm	76.21% <ø> (ø)

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[holgerd77]

Nice, super clean implementation and so much removed code, love this! 🤩

Will merge.