samples: common: simplify net_connect() usage and implementation

[mniestroj]

Checking for CONFIG_GOLIOTH_SAMPLES_COMMON=y is pointless, since all samples enable it in prj.conf. Call net_connect() unconditionally instead.

In case of CONFIG_NET_CONFIG_NEED_IPV4=y Zephyr networking subsystem initialization code already takes care about configuring IP layer and making sure there is assigned IPv4 address before reaching main() function. Make net_connect() a noop in that case, as no more networking setup is needed.

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