Revert "base: limit mtail.service memory"

Turns out MemoryLimit= includes swap memory, not ideal.

This reverts commit 0ed7d159.
1 job for master in 180 minutes and 2 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #8706
ai3
run_full_test

03:00:02

 
Name Stage Failure
failed
run_full_test Test The script exceeded the maximum execution time set for the job

Command: ["unregistervm", "cbc5edd2-5ed1-4600-bf52-c89b9dc51a6e", "--delete"]

Stderr: VBoxManage: error: Cannot unregister the machine 'test-full_host1_1559971194612_48116' while it is locked
VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
VBoxManage: error: Context: "Unregister(CleanupMode_DetachAllReturnHardDisksOnly, ComSafeArrayAsOutParam(aMedia))" at line 157 of file VBoxManageMisc.cpp
~/builds/20519861/0/ai3/float/test/test-full ~/builds/20519861/0/ai3/float/test/test-full ~/builds/20519861/0/ai3/float/test
./run-test.sh: line 38: popd: /home/gitlab-runner/builds/20519861/0/ai3/float/test/test-full: No such file or directory
ERROR: Job failed: execution took longer than 1h0m0s seconds