block: remove support for label

The "label" property was specific to i3blocks and implemented as an easy
way to prefix the "full_text" property. However it brings some
inconsistencies to the user, such as the calculation of the "min_width".

For this reason, The support for the "label" key is removed.
Below are examples how to properly add a label for a "time" block.

Either you can keep the "label" key and use its value in the command:

    [time]
    label=Time:
    command=echo -n "$label"; date +%T
    interval=30

Or more cleanly with a static block on its left:

    [time-label]
    separator_block_width=0
    separator=false
    full_text=Time:

    [time]
    command=date +%T
    interval=30

Refs #219

block.c

@@ -130,8 +130,6 @@ static int block_stdout(struct block *block)
 
 int block_update(struct block *block)
 {
-	const char *full_text;
-	const char *label;
 	int err;
 
 	/* Reset properties to default before updating from output */
@@ -143,18 +141,6 @@ int block_update(struct block *block)
 	if (err)
 		return err;
 
-	full_text = block_get(block, "full_text") ? : "";
-	label = block_get(block, "label") ? : "";
-
-	if (*full_text && *label) {
-		const size_t sz = strlen(full_text) + strlen(label) + 2;
-		char concat[sz];
-		snprintf(concat, sz, "%s %s", label, full_text);
-		err = block_set(block, "full_text", concat);
-		if (err)
-			return err;
-	}
-
 	/* Exit code takes precedence over the output */
 	if (block->code == EXIT_URGENT) {
 		err = block_set(block, "urgent", "true");