Move adb backup/restore helptext to adb shell bu help.

Key/value support for adb backup was added in ag/1946677, and help text
for adb shell bu is added in ag/2088937. This CL points the user to this
helptext instead of keeping it here, since it can quickly get out of
sync.

BUG=36170696

Test: adb
Change-Id: Ic40679f421a32e60f292ad20b726a89bfff03349
diff --git a/commandline.cpp b/commandline.cpp
index 4979eef..ed6693c 100644
--- a/commandline.cpp
+++ b/commandline.cpp
@@ -169,15 +169,7 @@
         "     '-k': keep the data and cache directories\n"
         "\n"
         "backup/restore:\n"
-        " backup [-f FILE] [-apk|-noapk] [-obb|-noobb] [-shared|-noshared] [-all] [-system|-nosystem] [PACKAGE...]\n"
-        "     write an archive of the device's data to FILE [default=backup.adb]\n"
-        "     package list optional if -all/-shared are supplied\n"
-        "     -apk/-noapk: do/don't back up .apk files (default -noapk)\n"
-        "     -obb/-noobb: do/don't back up .obb files (default -noobb)\n"
-        "     -shared|-noshared: do/don't back up shared storage (default -noshared)\n"
-        "     -all: back up all installed applications\n"
-        "     -system|-nosystem: include system apps in -all (default -system)\n"
-        " restore FILE             restore device contents from FILE\n"
+        "   to show usage run \"adb shell bu help\"\n"
         "\n"
         "debugging:\n"
         " bugreport [PATH]\n"