Revert "setting thread_pool_timeout is useless because vtc_mode overrides"

This reverts commit fa3e1419.

We might have over-bread such that thread_pool_timeout becomes relevant
at the other place in pool_herder()

bin/varnishtest/tests/r01490.vtc

@@ -9,6 +9,7 @@ varnish v1 \
 	-arg "-p thread_pool_min=2" \
 	-arg "-p thread_pool_max=3" \
 	-arg "-p thread_pools=1" \
+	-arg "-p thread_pool_timeout=10" \
 	-vcl+backend {}
 varnish v1 -start
 
@@ -23,16 +24,16 @@ logexpect l1 -v v1 -g raw {
 
 varnish v1 -cliok "param.set thread_pool_min 3"
 
-# thread_pool_timeout overridden by vtc_mode with delay=0.5
-delay 1
+# Have to wait longer than thread_pool_timeout
+delay 11
 
 varnish v1 -expect threads == 3
 
 varnish v1 -cliok "param.set thread_pool_min 2"
 varnish v1 -cliok "param.set thread_pool_max 2"
 
-# thread_pool_timeout overridden by vtc_mode with delay=0.5
-delay 1
+# Have to wait longer than thread_pool_timeout
+delay 11
 
 varnish v1 -expect threads == 2