Remove shared black/lint/lint-incr envdir in tox.ini`

Under tox3, it was faster to have the `black`, `lint`, and `lint-incr`
environments share an `envdir`.  However, tox4 recreates the environment
every time one switches from one to another when they share an `envdir`,
leading to much slower execution.  Here I've changed these environments
to all use the default (unique) directory rather than a shared one.

Closes Qiskit#9782

tox.ini

@@ -22,7 +22,6 @@ commands =
   stestr run {posargs}
 
 [testenv:lint]
-envdir = .tox/lint
 basepython = python3
 commands =
   black --check {posargs} qiskit test tools examples setup.py
@@ -36,7 +35,6 @@ commands =
   reno lint
 
 [testenv:lint-incr]
-envdir = .tox/lint
 basepython = python3
 allowlist_externals = git
 commands =
@@ -50,7 +48,6 @@ commands =
   reno lint
 
 [testenv:black]
-envdir = .tox/lint
 commands = black {posargs} qiskit test tools examples setup.py
 
 [testenv:coverage]