node: Remove redundant filepath.Join in parsePersistentNodes.

[eternnoir]

// StaticNodes returns a list of node enode URLs configured as static nodes.
func (c *Config) StaticNodes() []*discover.Node {
    return c.parsePersistentNodes(c.resolvePath(datadirStaticNodes))
}

// TrusterNodes returns a list of node enode URLs configured as trusted nodes.
func (c *Config) TrusterNodes() []*discover.Node {
	return c.parsePersistentNodes(c.resolvePath(datadirTrustedNodes))
}

Already use c.resolvePath to get full file path. Don't need join in parsePersistentNodes function.

[mention-bot]

@eternnoir, thanks for your PR! By analyzing the history of the files in this pull request, we identified @karalabe, @fjl and @zsfelfoldi to be potential reviewers.

[fjl]

Thanks!