Is the coronavirus getting less dangerous?
The issue is using the number of positive tests as a measure for the total number of active cases at a given time. At least in the U.K., the early testing capacity was much lower than it was now: 
Because of this, many people who were infected were not being tested and so weren't included in the total number of infections. Therefore, the total number of infections in April/May time was likely a large underestimate. It's been estimated that there was something in the range of 150,000-200,000 new cases per day in that time period.
On the other hand, testing capacity in the U.K. at the moment is much higher and will catch a much higher proportion of the total cases.
Therefore, your crude calculation of 'death rate' = deaths / cases is skewed by the error in the estimated number of cases, making it appear the 'death rate' is going down because we underestimated the total number of cases early in the pandemic.
The other thing to note is that the 'death rate' is dependent upon demographic factors, most obviously age. Early in the pandemic in the U.K., a large number of cases occurred in care homes, where the residents were particularly vulnerable. On the other hand, cases are high in students at the moment, who are a low-risk group. If the age distribution of cases is relatively skewed towards older people, then the 'death rate' will appear higher than vice versa, even though nothing may have biologically changed with the virus.
Note: Sorry, I don't have citations for a lot of this. I will try and find them and add them at a later point.