Remove workaround for missing prepared delete support in EXASOL about sqlalchemy-exasol

[vamega]

I don't think this will work correctly with turbodbc.
When I remove the self.isdelete from the pre_exec method in base.py, I get a MemoryError: bad allocation. I'll try to do some more digging and see if I can make a reproducible test case.

The current spot where I see this has a very large number of parameters.

[vamega]

Actually, I think this cannot be removed yet.
Reducing the number of parameters caused me to bump into this error:

Feature not supported: Prepared DELETE with parameterized condition other than <column name> = ? or unsupported type

My condition was to delete using an IN clause.

Using the current logic for statement construction, it appears that there is excessive allocation happening in the pre_exec, since it performs repeated string replacement.

I will provide a PR that changes this to use a template based single replacement, since that should dramatically increase performance when there are a large number of parameters.

[jank]

@vamega reading the EXASOL ticket https://www.exasol.com/support/browse/EXASOL-875

Currently, a DELETE statement may only have one prepared parameter.
We will allow multiple parameters, but only if they are no expressions and combined via AND, e.g. "col1=? AND col2=?".

only a special case for parameterized DELETE was implemented.